Bosses
Most regions have a mini-boss on stage 4 and a boss on stage 7; Spire Hill Crypt ends with its mini-boss and Spire Hill Crypt Lower with a boss on stage 3. Entering a boss stage needs a skull of the difficulty played, used up when the stage is cleared. The final boss is Lord of the Cinder on The Cinder Crown: 7.
Bosses and mini-bosses
Every mini-boss and the Tidewrath Crab King drop the skull of the difficulty played each time they are beaten (10 enemies; for online characters the game server plans the loot).
Enemy level 1-10
Enemy level 11-16
Enemy level 17-19
Enemy level 20-28
Enemy level 28-34
Enemy level 34-40
Enemy level 40-46
Enemy level 46-52
Enemy level 52-58
Enemy level 58-64
Enemy level 65-70
Mythic Rift
All 20 region bosses and mini-bosses above are the boss pool of the Mythic Rift, where they fight at level 70 with tier-scaled health and damage.