First, you’ll need to reach Chelona’s Rise. This is only possible after defeating Astel, Naturalborn of the Void, the large insectoid boss you’ll fight during Ranni’s quest. Astel is found in the Lake of Rot region near Grand Cloister. Once Ansel is down, this will allow the player to access the lift found in Deep Ainsel Well. That lift takes the player to the Moonlight Altar plateau. Chelona’s Rise is in the southwest corner of Moonlight Altar.
When you first find it, Chelona’s Rise is sealed off by a blue magic force field. A book on a pedestal outside prompts the player to find “three great wise beasts.” This refers to the three huge, blue-tinged tortoises found across Moonlight Altar.
Note: don’t fast-travel or rest until all three tortoises are defeated, otherwise they’ll disappear.
The first tortoise is found just behind Chelona’s Rise. From the entrance, head west to the cliffs and look over the edge. The tortoise will be clinging to the side of the cliff. Dispatch it with a ranged weapon or spell.
From there, head east along Moonlight Altar. Follow the cliffs until you reach a rock formation that leads to a higher plateau. Follow that plateau to the southeast corner, look over the edge, and find Tortoise #2 on a lower plateau.
Finally, travel to the northwest corner of Moonlit Altar, near Ringleader’s Evergaol. Look for the Spirit Spring. Tortoise #3 is floating at the top of it. Just hop on Torrent and ride the Spirit Stream upwards. Doing so will break the Chelona’s Rise seal. Find Ranni’s Dark Moon in a treasure chest at the top (via YouTube).