It is obtained by speaking to Kala during the "Betrayer of the Horadrm" quest, after getting Zoltun Kulle's head, and before speaking to Adria. Although the player speaks to her, the little blue star may not always appear above her head to notify you of a new 'talk' option.
It reads as follows:
When the desert sands silence their whispers in the night, and the thousand lights of the stars bleed into the darkness, I will find you in the void. My love will pour from me as words pour from my pen. - Kala