Alebus was a Hammerite MasterForger. Approximately fifty years prior to the events of Thief: Deadly Shadows, Alebus drank from The Builder's Chalice like that of Saint Edgar. He was unworthy and burned to death. Since then, all MasterForgers were barred from drinking from the Chalice.[1]
Since this happened around The Catastrophe, the ritual may have been done in response to the deluge of the Undead.