Project Floodgate

Left 4 Dead 2's campaigns, overrun by Halo's Flood. You and up to three friends drop in as an ODST squad and fight safe room to safe room through maps the Flood has started to grow over, with weapons and gear from across the Halo games. Everything Halo in it is read from your own copy of Halo: The Master Chief Collection when you press Play. Nothing from either game ships with the mod.

What you get

• The Flood replaces the infected. Common infected are the Flood's soldiers: combat forms of Halo 3's marines and ODSTs, whole, mauled, headless and ravaged. The special infected are the rest of the Flood, each a form of its own: the Boomer is Halo 3's carrier form, the Tank a tank form, the Hunter a stalker form, the Smoker a ranged form, the Charger a Brute combat form, the Spitter an Elite combat form, the Jockey an oversized infection form and the Witch, alone, a ravaged civilian host. Blow one apart and the pieces are the Flood's: tentacled arms, infected heads, legs and chunks of torso from the combat forms (a carrier's burst leaves the carrier's), falling and coming to rest on the ground.
• The maps are overgrown. Growths from Halo 3's Flood levels (mounds, piles, fleshy plates, tentacles, hanging strands, sacks and egg pods, bones) take hold where a growth would: along the foot of walls and piled into corners, up tall walls, round door frames, hanging from ceilings indoors, at the foot of wrecks and kerbs and along the verge of open ground. A street or a hotel floor carries one to two thousand of them, on any campaign, thicker the deeper into a campaign you are. They grow only where the squad can walk to and see: not on rooftops across the street, behind fences, or on the sky. They are scenery: nothing blocks the way or hides a pick-up, and the room you start in stays clean. Sacks and egg pods grow against the walls, never out where the squad walks. A colony nobody can see is switched off until somebody could, so the game draws and sends only what is in sight. Needs Halo 3.
• The air is the Flood's. Every map lies under a thick, dark, brown-grey gas, as Halo 3's Floodgate does: the map's own fog is turned that colour and drawn in close, the sky goes with it, the game is kept from brightening the dark, and clouds of the gas drift around the squad with spores floating in them.
• The survivors are ODSTs, or whoever you choose. Buck, Dutch, Romeo and Mickey for the Left 4 Dead 2 cast; Sergeant Johnson, the Rookie and two more troopers for the Left 4 Dead 1 cast. In the launcher's Squad page each survivor can be any of 33 Halo characters who carry a weapon, from whichever Halo games you have: the ODST squad, Dare, Johnson and the marines of Halo 3: ODST; Master Chief, Johnson, the Arbiter, an Elite and the marines of Halo 3; Noble Six, Carter, Kat, Jun, Emile, Jorge, John-117, an Army trooper and an ODST from Halo: Reach; the Master Chief, Johnson, Captain Keyes and the marines of Halo: Combat Evolved. Each comes with their own first-person arms and speaks their combat lines in their own voice (Combat Evolved's characters in Halo 3's voices: its Johnson's, its marines' and sergeants'). They stand, aim and run upright, the way the game's own survivors do: their armour bends with the survivor's own body, and neck and head keep the Halo character's own proportions. Their faces are on the HUD with their name across the foot of the picture (and on the picture shown while they are down, in the lobby and at character select), drawn from them, and a squadmate the game plays goes by their name ("Dutch", "Chief", "Six") on the scoreboard and in the game's messages.
• Every Halo weapon is a weapon of its own: 58 of them, from four Halo games. Each is a gun or blade with its own name, magazine (its Halo size), reserve, gunfire and quirks. The arsenal is in six tiers, each killing about a quarter faster than the one below, and many weapons have abilities of their own: fire, bleeding, stuns, knockback, rounds through every form, charged shots, a turret that spins up and overheats (the Loadout page says what each does). A plasma, needle or spike bolt, or a beam, is the shot itself: it hurts what it reaches, when it gets there. Every shot plays its Halo sound whole, the shots ringing on under each other as in Halo. No gun hits softer than the Left 4 Dead 2 weapon it rides, and each rides the one that moves most like it, and Floodgate's script makes it its own. Pistols, SMGs and Needlers, assault rifles, Spikers and plasma rifles, the Battle Rifle and the Carbine, marksman and sniper rifles, four shotguns, launchers from the M319 to the Spartan Laser, three heavy weapons, ten melee weapons with the Energy Sword and the Gravity Hammer, and Halo 3's flamethrower: each one is described in the README that comes with the download.
• Weapons to find on every map, by chance. The maps' own weapons are taken away and every map has its own weapon spots off the main path, each a weapon drawn at random: a gun six times in ten, else a melee weapon, the higher tiers rarer (tier 6 one time in twenty). They lean on walls, hang on brackets, lie on tables and floors, and glow in their tier's colour: grey, green, blue, magenta, orange and yellow for tiers 1 to 6.
• A shop in every safe room. Along its walls: a box for each weapon modifier, in its colour with its icon over it, to fit to the gun in your hands; every throwable and healing item; and five weapons drawn as the maps' are. Look and press use to buy with your score, at the launcher's prices; your launcher takes it off your record.
• Weapon modifiers, earned with each gun. Seven slots under each gun on the Loadout page take modifiers you unlock with your score and rent for that gun for a session: harder shoves, a laser sight, more reserve, bigger magazines, faster reloads, a faster rate of fire and harder hits, for you alone, on that gun of your kit (a copy you pick up on the way is plain). Two slots are open from the start; the rest open as you score with that gun.
• Weapon skins, earned the hard way. Four camos for every weapon, bought once with your score and worn by it for good: Platinum (once every modifier slot of the gun is open), Gold (Platinum earned on every weapon of its kind), Dark Matter, a drifting purple galaxy (Gold earned on every weapon), and the Flood: the gun overgrown with flesh, sacs and tentacles, its rounds trailing the Spitter's goo (2,500 kills with it once Dark Matter is earned).
• Your own kit, bought with your score. On the launcher's Loadout page each player picks a primary and a secondary from the whole arsenal, a throwable, a healing item (or an ammunition pack) and pills, and starts every chapter of the session with them, and has them back after dying and coming back; in co-op every player starts with their own. Every weapon and item is locked at first (the MA5C and the M6C are standard issue): unlock it once, for good, with the score you have earned, then buy it for a session (about a quarter of its unlock) to start with it. A session lasts until the game and the launcher close.
• Halo's campaign scoring. Every kill scores, and Halo's medals score more: multi-kills from Double Kill to Killionaire, sprees without taking damage from Killing Spree to Unfrigginbelievable, a medal for each of the Flood's special forms, for how one fell (Headshot, Beat Down, Sniper Kill, Grenade Kill, Incineration, Protector), for the Spartan Laser and the needle supercombine, and for finishing a chapter and a campaign; harder medals and harder difficulties score more. A Halo panel at the middle of the left shows the chapter's time, the difficulty by Halo's name, your score and the chapter's kills and medals, with each medal's picture as you earn it.
• Halo's energy shield. Every survivor has a recharging shield worth 25 health, as Halo 3's Spartan does: hits land on it first and only what it cannot stop reaches your health; five seconds after the last hit it starts to recharge, and it fills from empty in two (Halo 3's own timing). You hear it with Halo 3's own sounds, on your own game only: a soft hit on the shield and the rising tone as it fills; the screen flashes as it breaks (Halo's alarm while it is down is left out: in a fight with the Flood it beeped on and on). It is down while you are incapacitated or hanging from a ledge and recharges once you are back up. Squadmates the game plays have one too.
• Your service record. Every point and medal you earn, in every session, is kept on your PC. The launcher's Record page shows your total score, what you have to spend and every medal there is, each with its picture and how many of it you have earned below it; point at one to read how it is earned and what it scores.
• Halo gear. Health packs, biofoam, a regenerator and a power drain for the health items; ammo crates, fuel cans, fusion coils and grenade crates for what lies around; a combat knife.
• The ODST visor. The HUD is drawn as Halo 3: ODST's, in ODST's own gold read from your Halo: the visor's rim, your health bar with your shield as a Halo shield meter just under it, the weapons in framed slots with the rounds in the magazine and the reserve, each shown by Halo 3's own HUD drawing of it (playing alone, every weapon its own picture), the icons of the modifiers on the weapon in your hands beside it, and the score panel; the middle of the screen is left clear.
• Halo's sound, everywhere it matters. The Flood screech with Halo 3's combat forms' own voices and its stalker shrieks, loud enough to carry over gunfire, so a horde is heard before it is seen; the Witch wails like the tunnels of Halo 3's Floodgate. Between fights the maps sound like Floodgate and 343 Guilty Spark, and while the Flood has not found the squad, the Gravemind speaks. Every sound a combat form makes is the Flood's, and a round striking one lands with a short, wet thud of Flood flesh. Halo weapon fire and explosions, and the game's own music for its moments (horde drums, stingers, the Witch, the Tank, the finale) is Halo's; its background beds are left silent, so Floodgate's background music never plays over another piece. Everything the squad says is said by the Halo character each survivor is, in their own voice and their own words from the Halo games. Every sound is levelled to sit with the game's own, so a horde fight stays clean.
• Halo's music all the way through. Under every map plays Halo's music, one piece after another, chosen by where your survivor is: ODST's and Reach's menu themes in the safe rooms, the Halo theme under the open sky, Combat Evolved's horror pieces under a roof, darker pieces on the last stretch and battle music in the finales; it steps aside for the game's own fight music (Halo's too). On Dark Carnival's finale stage the band plays Breaking Benjamin's Blow Me Away, read from your own Halo 2, or a song of your own chosen on the launcher's Gear page.
• The menus are the Flood's. The main menu shows your squad deep in a place the Flood has taken, Halo 3's biomass on every side and a horde coming out of the haze, under a Floodgate title, to Halo 3: ODST's menu piano; the end-of-chapter screen, the game-mode tiles and the special infected's pictures are the Flood's too, all made on your PC.
• The Flood's own rules on top of Left 4 Dead 2's.
  • The dead get back up. A fallen combat form leaves its body behind, the very form that fell, and troopers walk over it without stumbling. Infection forms come for it in packs of three to six, running in from some way off and out of the squad's sight where they can (a combat form never bursts into infection forms itself), weaving round each other: a pack finds the nearest whole body, one latches on and raises it a few seconds later, and the rest go on to the next. Only with no body anywhere near does a pack run at a trooper, and a form that reaches one bites for the least the game counts and bursts. Shoot, swing at or bash an infection form and it pops, and the ones around it go with it (a swarm popping at once is heard as two pops, not a burst of static). Shoot, swing at or bash a body and it is torn apart, silently but for the tearing itself, and a torn body can never rise again.
  • Twice at most. A combat form can be raised twice; killed a third time, it comes apart.
  • Blades and blows tear. A combat form killed by a melee weapon, a shove or a gun's butt is torn apart where it falls.
  • Carriers burst into a swarm. A carrier form bursts into 12 to 18 infection forms.
  • Sacks and eggs burst too. The Flood sacks and egg pods in the colonies burst when they have taken three assault rifle rounds (a shotgun blast, a sniper round, a blade, a bash, fire or a blast does it at once), leaving a burst husk (an egg cracks open) and a pack of 4 to 7 infection forms. A round stops in the first sack it hits.
  • The colonies breed. Now and then a pack crawls out of a colony near the squad. At most 90 infection forms are out at once.
  • Fallen troopers turn. An ODST who dies comes back as a Flood combat form, outlined in red and far tougher than the rest, that hunts the rest of the squad.
  • The Flood is hard to kill. Every Flood form has twice the health it has in Left 4 Dead 2 (a combat form four times a zombie's), and half as much again for each step of difficulty above Easy: one and a half times that on Normal, twice on Advanced, two and a half times on Expert.
  • No friendly fire. Nothing the squad fires, throws, burns or swings hurts a squadmate, on any difficulty; only a trooper's own blast can hurt themselves.
• Ammunition is tight, and every weapon pulls its weight. Every gun's magazine is its Halo size, and it carries twice its Halo reserve; pistols have unlimited rounds. Every gun hits at least as hard as the Left 4 Dead 2 weapon it rides, and harder the higher its tier.

Playing

• Needs: Left 4 Dead 2 and Halo: The Master Chief Collection with Halo 3: ODST installed, both on Steam, Windows 10 or 11.
• Better with more Halo: Halo 3, Halo: Reach and Halo: Combat Evolved are each used when they are installed in your Master Chief Collection. Without one of them, its weapons fall back to their ODST counterparts and, without Halo 3, the maps stay clean. Halo 2 brings Blow Me Away and the cutscenes behind the launcher's title; without it the concert stage plays the Halo theme with guitar.
• Alone or together: single player with three bot squadmates, or co-op for up to 4 through Left 4 Dead 2's own lobby. Every player needs the mod. The host must choose Local Server in the lobby, because the Flood rules and the growths run in the host's game. Join through the game: the host starts a campaign from Left 4 Dead 2's Play Campaign menu with Local Server chosen in the lobby; friends join from their Steam friends list (Join Game) or the invite the host sends. No server of Floodgate's own is needed: each player's kit, options and characters are read from their own PC by their own game. If joining fails with a Steam validation error (the host's console says the player's ticket was invalid), Steam itself refused the game's sign-in ticket: quit Steam on the PC that was refused (and on the host's, if it keeps happening) and start it again. Floodgate's launcher sees that error in the game and tells you so.
• First start: a small window builds the mod from your games (a few minutes with every Halo title installed). After that it starts straight away, and rebuilds only when the mod, one of the games or your options change.
• Loadout and squad, from the game's main menu: while the game runs, press F9 (the main menu says so under Quit) and Floodgate's own window opens over it, overgrown like the rest. Loadout: your starting kit, a primary and a secondary taken from the whole arsenal, listed by kind with a picture of each weapon drawn from your own games, its handling as bars (damage, rate of fire, magazine, reserve), its seven modifier slots and its mastery, its quirk, and where it sits in your hands (right/left, up/down, forward/back) and how big it is (50 to 150 percent), with a small picture of it in the arms of one of your squad's characters that follows the sliders; a switch under them (Turn instead / Move instead) makes the same three sliders turn it round those axes instead, up to 180 degrees each way (tilted up or down, turned right or left, rolled right or left, round its grip). Beside it, Hands on / Hands off shows or hides the first-person arms: off, the picture and the game show your weapons with no hands holding them (it takes a build, as a change of character does). Gear: a Halo model for each throwable and healing item, with the same sliders and switch. Squad: each survivor's Halo character, with a picture of them as the game will show them. Record: your service record and medals. The score you have to spend shows at the top of every page. Press Save and Floodgate offers to restart the game with your choices: it closes the game, builds them from your own games (a few minutes) and starts it again at the main menu, so you load into the level as your chosen characters with your chosen kit. A change of kit alone needs only the restart, not a build. The same window is also a button away on Floodgate's start window, and Floodgate.exe options opens it on its own.
• The start window: pressing Play in Melty opens Floodgate's own small window first, and it waits for you: Play starts Left 4 Dead 2, Test Range starts it on the test range, Loadout and Squad opens the Options window and comes back here. Closing it starts nothing. The launcher's windows play Halo's cinematics behind the Floodgate title (read from your Master Chief Collection the first time, then kept in Floodgate's own folder), with Halo's cut-cornered buttons and its pages in columns of dark glass edged in gold.
• Leaving no trace: the addon is put into Left 4 Dead 2 when you press Play and taken out again when the game closes, so Left 4 Dead 2 started from Steam is the game you know. The same goes for the small stand-in files that set the main menu's video aside (in the game's update\media folder; the game's own videos are never touched). If the launcher is ended by force (Melty's Stop, say), the game closes with it; the addon and the stand-ins are then left in place until the next Play.

The README that comes with the download lists what does not work yet (Known limits).

Credits and license

MIT license for the mod's own code, sheets and scripts (see LICENSE). Built with Claude.

• Sound decoding by vgmstream, bundled unmodified under its own license (see third_party/vgmstream/COPYING).
• File format knowledge from the Halo modding community's public documentation, in particular the Assembly and Reclaimer projects. No code from either is included.
• Left 4 Dead 2 is Valve's. Halo is Microsoft's and 343 Industries'. This is a free fan work, not affiliated with or endorsed by either, made under Microsoft's Game Content Usage Rules and Valve's rules for free mods.