After speaking with Shanrah, it is faster to find the Forgotten Guardian at the northwestern end of Crystal Overlook and have him teleport you to the Arid Sea than to map travel back to the Crystal Desert. Rotal's bones are right next to where the Guardian sends you. After speaking with Rotal, you will need to kill Kepkhet Marrowfeast in Prophet's Path. See that article for her exact location as well as useful tips. In general, watch out for the trappers that surround her, and try to bring a way to either daze her, or penalize her for spamming spells (such as Backfire).