From ThornsWiki

A diablerie is an uncontrolled spell that manifests at random in a passive. The spell that is unleashed is almost always extremely powerful, and is often dangerous and destructive. Though extremely rare (a diablerie usually only happens once or twice in a passive's life), the effects of a diablerie can be deadly, even to the passives who "cast" them.

Current (published) research suggests that diableries can occur when a passive is stressed, hurt or traumatized. In reality, there is not always a clear explanation for what causes a diablerie. Sometimes they seem to occur at random.

The galdori consider the potential danger of a diablerie to be justification for "gating" (or enslaving) passives to the University of Brunnhold, where they can keep a close eye on them.