Diablo

Blizzard North took the dungeon RPG, where moves are made in turns, and removed the turns. The character walks and strikes wherever a mouse click points; control is reduced to one hand, and the barrier to entry drops almost to zero. The idea started out with turn-based combat, and the switch to real time changed not only the pace but also what people come back to the game for.

The dungeon is reassembled from ready-made fragments on every visit, so the map can't be memorized and scouting stays real work. An item isn't fully defined either: a base type is taken and properties with random values are added to it, so two swords under the same name turn out to be different things. Out of these two generators grows a pursuit the genre hadn't had as a separate thing before: you play through the game for the next find, and after the ending you go in again. Co-op play ran through Battle.net, a service launched together with the game, and joining someone else's game needed neither an address nor a prior arrangement.