A better question is how is he tormenting the Evanuris.
If the Evanuris are simply locked away I can't see that as harsh. Logic. And if the Old gods did truly whisper to the mages of Tevinter what ever influence they are under is implied to be a suppressing agent that is potentially weakening.
Speculation: The Old gods called the Tevinter mages to the Black City to test just how much what ever mechanism traps them had weakened. Secretly they knew the Tevinter magisters would become blighted and their trap would strengthen thus stopping them from whispering. However, their plan was long term. Having seen or tasted power some other fool if not Corypheus could devise a means to undo their predicament.
The same millennia could at the very least apply to the punishment of Evanuris give or take some years within the thousands. I would be surprised if humans had been the only contacted beings by the Old gods.