After Bayer discovered his own problems, Zariel knew his days were numbered. No matter how cautious Bayer was, he would continue probing, and soon he would uncover all of Zariel's plans.
Zariel decided to take the initiative.
While Bayer was still wrangling with Hircine, Zariel had already infiltrated Blood River Fortress.
Thanks to his familiarity with the fortress and the various permissions still in his hands, Zariel's infiltration was silent. Not until Hircine and Bayer fell out and Harvey exploded out of the furnace did Zariel finally find Bayer's weakness.
He took advantage of the moment Bayer was distracted and launched a successful surprise attack, smoothly sealing him.
But the instant Zariel made his move, he transformed completely into an angel and could no longer return to his past form.
The laws of the First Layer of Hell would soon begin to target him as an intruder, either slowly corrupting him back into a devil or killing him outright.