From the Steam store page:
Vermintide is a co-operative action first person shooter and melee combat adventure set in the End Times of the iconic Warhammer Fantasy world.

Vermintide takes place in and around Ubersreik, a city overrun by Skaven. You will assume the role of one of five heroes, each featuring different play-styles, abilities, gear and personality. Working cooperatively, you must use their individual attributes to survive an apocalyptic invasion from the hordes of relentless rat-men, known as the Skaven. Battles will take place across a range of environments stretching from the top of the Magnus Tower to the bowels of the Under Empire.