Approach The Mother and pay attention to the scene. Time for the final boss fight.
BOSS - The Mother
The Mother will cast Glyph of Neutralization on top of the Giant Tentacles rendering them immune to magic and draining you of mana. If your friends begin to fall you can revive them with the Tower of Healing. The best way to avoid devastation is run to the back of the cave and fight the Tentacles there. Use the Tower of Trauma immediately to stun them and take them out. Use the Tower of Flame to aid you on the Tentacles and when the Childer Grubs appear use the Architect’s Pyre in conjunction with area of effect spells to neutralize the enemies. Once The Mother is the only enemy up simply use your melee to damage her and your mage(s) to heal and AoE the Grubs that appear. When she picks up a player be sure to freeze her to stop massive damage from being incurred.
Completed Dragon Age: Origins - Awakening ('Dragon Age: Origins - Awakening' expansion).