Lumalia's long imprisonment deeply affected her (she also changed from neutral good to chaotic good). She regretted all the time she spent in stasis. Lumalia wanted to bring justice to the Thayans for their crimes but in the fury of battle she could succumb to a thirst for revenge.
Centuries before, the Red Wizard Kazit Gul had summoned Lumalia and bound her to the Doomvault. He put her in stasis and compelled her to slay each time she awoke. She sensed the death of Mystra during Spellplague and succumbed to despair.
Awoken in 1486 DR at the arrival of heroes from Daggerford, Lumalia sensed something different, she felt the celestial spheres shifting. With this, Gul's power over her was broken. She offered her aid to the heroes.