When you enter an area, the game checks your level and draws a creature from the following list that is less than or equal to your level.
Level 1: Ash Slave
Level 8: Ash Zombie
Level 15: Ash Ghoul
Level 25: Ascended Sleeper
Level 3: Ash Slave
Level 10: Ash Zombie
Level 17: Ash Ghoul
Level 27: Ascended Sleeper
Level 1: Ash Slave
Level 6: Ash Zombie
Level 13: Ash Ghoul
Level 23: Ascended Sleeper