For Act 4. during one of random encounters you'll get a Thanatonic rune  (misc item, not a quest one)
You need to place it in four slots in various parts of the city. Prepare for brutal fights. The correct order is:
1 - Lower City, south from the elevator, Religious check on a wall;
2 - Middle City, east, Shamira's isle, Aasimar's house (you know which one), northern wall;
3 - Upper City, north east, former Woljif's quest house, take a portal to the balcony;
4 - Middle City, go left from Shamira's palace, there's a barely visible alley there that leads to its backyard.