Astral Corruption

This horrible ritual is the cause of Blood magic being banned in ancient times. With an awesome sacrifice, the fabric of astral space becomes tainted with blood. All spell casting uses the tainted Arcana and attracts the attention of Horrors. Every time a non-Blood magic ritual is cast, a magic item is forged or a mage is empowering himself, there is a chance that a Horror will follow the arcane flow and attack the mage. The more gems spent the greater the chance of attracting a horror.

Text: Illwinter Game Design, via Dom6 Inspector