Grendel

Grendel was an alchemist who'd been in a little magical accident.

When the Avatar found a talking giant rat in a hut south of Cove in Ultima V, the hero's confusion was great for a moment. As the rodent soon explained, it turned out that the alchemist Grendel had tried to discover a spell to turn beings into rats. The resulting enchantment, however, worked too well -- transforming him permanently into a giant rat. Still, he managed not to drown in despair and, after some refinement, perfected a version of the spell that would turn another being into a rat, only temporarily.

He was willing to share the secret with the Avatar, sying that the 6th Circle spell would need Spider Silk mixed with Sulfurous Ash and the incanation 'REL XEN BET' to put the spell into effect.