You could use the premise of every classic FPS. A group of wizards trying to set up a Tippyverse style network of teleportation circles did something wrong and accidentally let something bad in from outside of their plane.

Or you could subvert the classic start in a bar. They start in a tavern, but the tavern is in the abyss and the characters are its demon patrons. And the "adventure" is just the everyday violence and brutality of the abyss. They're doing ordinaryish errands but what would be minor conflicts elsewhere instead escalate into bloodshed and dismemberment (like in the game Postal 2, especially the later stages)