Grendel, from Ultima V: Lazarus
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 during the time of Ultima V. 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, if only temporarily.
 Ultima V: Lazarus
In Ultima V: Lazarus, the well-spoken Grendal revealed to the hero that before he was transformed into a giant rat by a magical accident, he had been a powerful alchemist. The creature was remiss that he was unable to offer a visitor the usual refreshments, for he now survived off a diet of swamp water and frogs. Grendel had been working on a spell called Vas Rel Xen Bet, which was designed to turn ones foes into rats, but the enchantment had gotten out of control and he was permanently transformed. Since that time, he had perfected a lesser version of the spell known as Rel Xen Bet. 
Grendel told the hero that the swamp he now lived in had once been a mining camp, but the area had become flooded. Lately, he had begun to sense an air of evil to the north-west, which if Jaana were in the party, she would confirm that she also sensed these emanations from the north.