Round-based Black Ops III Zombies in Backrooms: Escape Together's levels, played in Garry's Mod.
Four levels in Backrooms' order: Level 0, Level 1, Level 3, Level 2.
Beat each level's round, finish its Backrooms objectives and solve its puzzle (VHS code, breakers, fuses, pressure dials) to escape.
Lights Out and the Alarm: Smilers in the dark, and an unkillable Bacteria that hunts you until you hide in a cubby.
Play the numbered hazmat survivors with emotes, a whistle and the camcorder.
Black Ops III music, announcer, perks, Mystery Box, power-ups and zombie sounds.
Start menu with room codes, and proximity voice chat.
Needs Garry's Mod, Backrooms: Escape Together and Black Ops III on Steam. Nothing from either game is included: the first Play converts what it needs from your own copies (about a minute, once).