The implication of the closing text as I read it was that Illidan was physically standing guard over the prison as its warden - not as host to Sargeras' essence or anything, but more empowered by the Pantheon's might to contend with Sargeras essentially forever to prevent him from leaving the Seat of the Pantheon. To achieve that basically both Illidan and the Pantheon have to be sealed away, leaving Azeroth as the sole Titan world-soul left in the physical universe - and, inadvertently, placing her directly in the sights of the Void as the final prize to turn the tide in the future Light vs. Shadow war.