[Deadnature]

Good question:

I first thought that all the slaves disappeared in the same instant that he cast his d-spell, but upon double-checking it seems the truth is that other bloodmages used his slaves to cast their own spells right before the mage in question could use them...he was relatively far away from the other blood-mages but there were a lot of body-guards crowding the place, so I guess that some of his slaves got "lost."