Seren can be found in the Tower of Voices in Prifddinas after the Light Within quest.
Notes:
During the Light Within quest, you help put Seren back together as she shattered herself because she didn't want to leave her elves when Guthix forced all Gods out of Gielinor.
During the Children of Mah quest Seren will help you navigate Mah's dreams. At the end of the quest, Seren will kill Mah to put her out of her misery.