On the way to the second area of the dungeon, there's going to be a room that will lock itself and you won't be able to leave it until you're destroyed the construct in the middle. The Overseer is an elite monster that you have to defeat In order to proceed to the next area of the dungeon. After slaying them all, an Overseer will drop down. You're going to have to walk around to find them but it should be fine as the dungeon is not that large and the enemies are just waiting around the corner. The first objective in the dungeon is to slay all of the monsters in the first area. As such, make sure to always keep an eye on your health so as to not fall for this. Like all dungeons, there's going to be a lot of monsters in here in such a way that being mobbed is very likely the way you're going to perish. This dungeon is filled with cultist enemies and werewolves. Nostrava is the first city you end up in and is to the southwest of Kyovashad. This dungeon is located in Nostrava inside the cathedral.

Diablo 3 cultist page 7