Go to the Shards of Myzrael, summon her and defeat her. Gather her Eldritch Shackles and bind them to the Shards of Myzrael.
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.