Aretha is an NPC located near the soul altar. She is the target for the Clue Scroll Anagram: "A HEART", The challenge scroll answer is 2.
To find Aretha, simply travel to Zeah via the southernmost port of Port Sarim, then Run North to the Dark Altar via Arceuus house, lastly, run east of the Dark Altar to get to the Soul Altar. This can be done from level 3.
Players who speak to her will learn that the river contains the souls of the dead of Great Kourend, and that they are guarded by Cerberus.