The Angel of Death was not sealed within any space that Thane or the rest could see or go to. It was sealed inside a separate dimension where its malice and whispers could not reach the hearts of men.
The dimension was sealed within the sigil, and it was the holy powers of the Temple that guarded it.
Since the start, Thane had suspected this from the Bible notes of several occasions where angelic beings were sealed in places, like the river Euphrates.
But now, they fully unsealed it, and the malevolent dimension began to spill out.
The entire temple was starting to crumble as the Holy Energy that contained the dark dimension had been broken.
Thane's eyes saw it.
The magic that spilled out from the platform and into the world. He knew it would burst, and ordered Sayla to protect them.