Either Cryptomancer Nasrara or Investigator Dahno, on behalf of their respective factions, have sent you to clear out the recently unearthed Dire Hollow. The monsters found within are far too dangerous to face alone, and as such you are advised to bring fellow adventurers with you.
The objective is to kill everything in sight. There are three different themes of monsters that are chosen randomly each time you enter: undead, mist grinnins, or treants. All three themes drop the same loot. Named monsters can also spawn, but they are not guaranteed to appear.