- "Thaumogenesis is when doing a spell actually creates a being. In this case it was, like, a side-effect, I guess. Like a price."
- ―Willow Rosenberg[src]
Thaumogenesis was the process through which a new being is created as a by-product of a spell. Usually the resulting creature was a demon.
When Jonathan Levinson performed an augmentation spell to alter reality and make himself the best at everything. This spell had the unfortunate side effect of creating a demon which when killed, broke the spell and reverted the world to its natural state.
Two years later, After Willow Rosenberg performed a spell to resurrect Buffy Summers, a demon that possessed Anya Jenkins, Dawn Summers, Xander Harris and Buffy Summers briefly was created. Willow and Tara Maclay performed a spell to make the demon corporeal, so that Buffy could fight and kill it.
Angel and Lilah performed a dark spell in the Hyperion Hotel lobby to turn the demon Sahjhan into a corporeal being and force him to open a rift in reality to enter Quor'toth. Weeks later, some sluk demons appeared in the Hotel they surmised that the demons were created via thaumogenesis. When other demons and two people began emerging from the rift, they realized that thaumogenesis was not at play.