Blister Rat Fiend.
Compared to an entire building, a human would appear quite small.
Even with all his accumulated power, Cain found it hard to beat the constraints of architecture. If he ran through it, he would be done in seconds, but he wanted to take his time.
He was very sure no one would come looking for him, as they all probably believed he was still locked up in that room. However, their lack of understanding of his abilities made sneaking in and out very easy to do.
In reality, they had every right to be lax. The power of the dome was supposed to completely suppress his magical power. However, it was unable to do so.
Hunter's ability could still be used because his power was not accessed through mana and the spiritual plane, but rather the Aether realm.
Cain walked through the darkness, completely relaxed.
Occasionally, he would encounter a critter. Putting them out of their misery was a more humane thing to do.