ULTRADOOM

ULTRADOOM drops V1 into Doom. ULTRAKILL plays as usual — your movement, dashes, slides, revolver, coins, rockets, parries — while the world around you is a real copy of GZDoom running Freedoom's E1M1, rendered from V1's eyes in real time and drawn behind ULTRAKILL.

What you need: only ULTRAKILL. You don't need to own Doom — the free Freedoom game data and a patched GZDoom come in the package.

How to play

• Start ULTRAKILL and open the Sandbox: Doom mode starts by itself and you're on E1M1.
• F9 switches Doom mode off and on, F11 puts you back at the map start.
• Punch doors, switches and lifts to use them (Doom's "use"). Keys are picked up by running over them.

What works

• Doom's monsters are run by Doom (AI, animation, sounds) and are real targets: every ULTRAKILL weapon, coins, explosions and parries hurt them, kills go on the style meter, and their blood heals you.
• Doom's monsters shoot and bite V1; Doom's items heal you, give overcharge armor, berserk punches, invulnerability.
• Doors, lifts, switches, keys, nukage, solid pillars and barrels — with Doom's own light, flicker and darkness on V1's arms.
• Doom's sound and music, following ULTRAKILL's volume. Pausing ULTRAKILL pauses Doom (hit stops and parries too).
• ULTRAKILL's own enemies (Spawner Arm) walk Doom's map, hide behind its walls and are lit by Doom's light.
• Cheats: Enemies Attack Each Other makes ULTRAKILL's enemies and Doom's monsters fight; Enemies Ignore Player and Blind Enemies work on Doom's monsters too.

Known limits

• Only E1M1 for now (the exit stops Doom mode).
• Doom's doors close on ULTRAKILL's enemies.
• ULTRAKILL's projectiles and blood are not darkened by Doom's light.
• Other enemy cheats (Invincible Enemies, Kill All Enemies…) don't reach Doom's monsters yet.

Single player. Plugin code: MIT (remixes welcome). GZDoom: GPLv3, source of the fork included as a patch. Freedoom: BSD-style license.