22 years later, this classic Half-Life zombie survival mod shambles onto Steam with all new features

Before Valve reinvented cooperative gaming with Left 4 Dead, there was Zombie Panic! A mod for the original Half-Life, Zombie Panic! pitted groups of players against a single, player-controlled brain-muncher. Which doesn't sound all that challenging, except that any survivors killed by the zombie would rise again as zombies themselves, slowly turning the tide against survivors until there was only one sorry human remaining.

Zombie Panic! was released on ModDB back in 2004—relatively late for an OG Half-Life mod.