This achievement is gained upon defeating the following rare monsters in Sanctum Somnium:

an arcane warden
Archmagus Erlen
Archmagus Nesalie
The Beast King
a bladedancer guardian
a bright warden
Conjurer Nallen
Demonologist Sharra
The Dream Collector
The Dream Destroyer
Executor Bashka
a flesheating beetle
a giant warlord
a goblin raid leader
High Priest Casmion
High Priest Darsia
an infected rat
Knight Captain Elena
Knight Captain Rosch
Larrow the Demented
a lifeweaver servant
Master Thief Quentin
a psychotic leprechaun
Rites Master Lorett
a shadow warden
a shieldbearer guardian
a soulmage servant
a spellsword guardian
a spellward servant