Gauntlet

Gauntlet arrived in arcades in 1985 from Atari Games, designed by Ed Logg, and defined the cooperative dungeon-crawler genre for the coin-op era. Up to four players simultaneously picked from four characters at one cabinet - Warrior, Valkyrie, Wizard and Elf - and fought through mazes full of monsters, treasure and food, which was the only way to restore health. The cabinet's voice narrated events with lines like "Elf shot the food" or "Wizard needs food badly," skull-shaped generators kept pumping out endless waves of monsters, and the game could theoretically run forever as long as players kept feeding it coins. That emphasis on cooperation, along with a fair amount of sabotage among players sharing one screen, made Gauntlet one of the most influential arcade titles of the mid-1980s.

The sequel, Gauntlet II (1986), expanded the monster generators, added new enemy types and levels, let players design their own mazes, and rebalanced the characters without changing the core cooperative formula. Gauntlet: The Third Encounter (1990) brought the concept to the handheld Atari Lynx, which couldn't offer multiple players around one screen, so the game worked more as a solo take on the original idea, wrapped in a simple story framework.

Later sequels and reboots of the series (Gauntlet Legends, Gauntlet: Dark Legacy) only arrived a decade or more afterward and moved the concept into 3D - it's really those first two 1980s arcade games that are still cited as one of the foundations of the multiplayer hack-and-slash dungeon crawler genre, and their influence still shows in much later co-op action games.