Abyss Survivors

Most survivors-likes are a solo trance. This one seats up to four people at the same keyboard: Arrows/WASD, IJKL, Numpad, TFGH. Everyone shares the screen, enemy HP scales with the party, and the last one standing carries the run. Solo works the same, minus the shouting.

Under the swarm it's the full loop: 12 survivors, 36 weapons that evolve into their upgraded forms, 22 passives, 5 realms, 42 achievements. And a Corruption dial you push yourself — higher Corruption means nastier enemies and richer drops, so you set your own risk. No install, no account, runs in the tab. (Online co-op is off in the browser build, it needs a server; same-device co-op needs nothing.)