From Dungeons of Dredmor Wiki
Effect: Drain Mana 13%
Necrodrain is a detrimental effect (Curse) that affects characters that cast spells such as Nightmare Curse and Tenebrous Rift. The drain is typically done to the caster in the form of a DOT with varying durations, based on the original spell cast.
It is a side-effect of spells cast by characters with the Necronomiconomics skill tree.
It is not currently known exactly how this drain effect works.