You must defeat Myzrael before she grows in strength. If you do not, then once she has gathered enough power she will challenge her captors! There are some who think they can control her, but if she can be controlled then why did the giants chain her in the first place??
Crazy logic, eh?
To defeat her, summon her at the Shards of Myzrael where you first spoke with her. Defeat her and gather her Eldritch Shackles, then bind them to the Shards of Myzrael.
Do that, and she will again be trapped.
As the eldritch shackles are placed around the Shards, an unhuman wail is heard from far below. The scream is followed by a roar.
The Shards of Myzrael no longer invoke feelings of warmth and security as they once did. Now they pulse with an open menace... as if the thing beneath them holds ill will toward the surface.