- "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 was created as a by-product of a spell. Usually the resulting creature was a demon. This acted as a universal price for having the casters will done, or as Anya Jenkins described it "a gift with purchase".
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 was meant to balance the effects of the spell by acting as the beast of everything. When Jonathan killed it, he 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. They discovered that the demon possessed them because it was out of phase with this dimension, meaning it lacked a corporeal body. Banishing the demon would only reverse the spell that created it, but the demon would eventually dissipate unless it killed Buffy, which would allow it to remain in this dimension. Willow and Tara Maclay performed a spell to make the demotn 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.