Oh, it was one of Arthur's former colleagues Bill was ordered to cruciate -- Cresus Deverill? I remember Arthur told us something bad about the man but I can't remember what.

Apparently he attracted Corax Mulciber's ire for filching some quills out of Selwyn's office, when MLE was done in there. Although my guess is he wanted to make an example of someone and didn't much care who or why.