A creature created when a wizard attempts one of sever forbidden divination rites and it fails catastrophically. The spell essentially rebounds back to them and turns them into this creature.
Session 004: Desecration over time via nothic transformation
Desecration can happen gradually over time in the case of a cleric, paladin, druid, or wizard that has been specifically consecrated by the Church becomes a nothic. In the event of a non-consecrated wizard turning into a nothic, desecration does not happen.