Tip: To save time, get this quest after doing the "被遺忘者" quest which leads you to Sariss.
You can reach the Arid Sea from a few points shown in the map. You can start from 命運峽谷, 占卜之石 or 絕望沙丘. Once in the Arid Sea, do not go for the star on your map. Siliss Yassith hides on top of a rocky mound with apparently no direct way up. When you approach the area from a distance you can see a giant statue to the east of the hill. When you get closer that giant statue collapses. The torso of the statue ruin now forms a bridge to the top of the hill. Cross this bridge to climb the hill. (Follow the dotted path on the map.) Beware there is at least one 沙巨蟲 in the area.