how random is Transformation?

[Zooko]

My Pretender God, Bphlah the Firey Head Baphomet, had empowered himself four times, Wished for magical power, and forged himself a Ring of Wizardry. So he had Astral-10, Fire-10, Nature-4, Water-4, Earth-4, Death-4, Air-2, Blood-2.

He cast Transformation, and the next turn I was informed that something had gone horribly wrong and Bphlah died during the attempt.

Okay, so suppose I Call God to get him back, then forge another Ring of Wizardry for him, then Wish for magical power again. Then should I try Transformation again, or is this going to happen every time?

Thanks.