Here is an overview of the game, via its Steam page:
Fight together with your friends against hordes of enemies in this new Warhammer 40,000 experience. From the developers of the best-selling and award-winning co-op action franchise Vermintide, Warhammer 40,000: Darktide is a visceral four-player co-op action game set in the hive city of Tertium.
In the depths of the hive, the seeds of corruption threaten an overwhelming tide of darkness. A heretical cult known as the Admonition seeks to seize control of the planet Atoma Prime and lay waste to its inhabitants. It is up to you and your allies in the Inquisition to root out the enemy before the city succumbs to Chaos.