Commonwealth Survivors

Left 4 Dead 2, fought by four of Fallout 4's companions, who look and sound like themselves.

• Hancock replaces Nick, MacCready Ellis, Cait Rochelle and Preston Garvey Coach. Each wears their own Fallout 4 face, hair, outfit and hat, built onto the survivor's skeleton, so every L4D2 animation and hitbox still works.
• They speak their own Fallout 4 lines, picked to fit the moment: reloading, spotting a Tank, getting pounced, going down, healing a friend, friendly fire, and the campaign chatter in between.
• Common infected snarl, scream and die like feral ghouls; the Tank roars like a Deathclaw; Tank fights play Fallout 4's boss combat music.
• Play solo with three bots, or co-op with up to 4 friends (everyone needs the mashup to see and hear it).

Join through the game: in Campaign, the host picks "Play With Friends" > "Create New Campaign Lobby", sets "Server Type" to "Local Server", picks "Create Lobby" and invites friends; they accept the Steam invite. Everyone playing needs this mashup from Melty, played once first.

You need both games installed. The first time you press Play, the mashup reads your own Fallout 4 (its voices, sounds, meshes and textures) and builds an L4D2 addon from them (about a minute and a half, about 1.1 GB on disk). Nothing from either game is downloaded with it.

Not in this version: first-person arms (still L4D2's), Fallout 4 models for the infected and the Tank, and the bots' name tags (L4D2 fixes those).

Credits: Fallout 4 by Bethesda Softworks and Left 4 Dead 2 by Valve, read from your own copies. Audio decoding by FFmpeg (LGPL 2.1, bundled unmodified). Mashup code and design sheets under the MIT license; open for remixes.