There are a total of 7 spells which count towards the Legendary Sorceries and Incantations achievement. Four of these are Sorceries, and three Incantations. This page details how and where you can find all of these legendary spells to unlock the achievement.
Where to Find All Legendary Sorceries
The Comet Azur legendary sorcery is perhaps one of the easiest to find once you reach the Altus Plateau / Mt. Gelmir region. All you need to do is find an NPC known as Primeval Sorcerer Azur, who is located near a cliff edge just east of Volcano Manor, or north of Hermit Village.
You can reach the area by riding north up the hill from Erdtree Gazing Hill Site of Grace, then through the ravine west of Wyndham Ruins, which brings you to the base of Mt. Gelmir. Continue north and then west through the ravine, bringing you to the slopes of the mountain. From here you can head south-southwest towards Hermit Village. There will be a Site of Grace here which is actually referred to as Primeval Sorcerer Azur, and Azur is just south of it. Once you reach the area, interact with Azur to obtain the spell.
Stars of Ruin
This spell can only be acquired once you’ve interacted with Primeval Sorcerer Azur, noted above. You’ll need to speak with Sorceress Sellen to progress her questline by showing her Azur’s sorcery. You can then ask Sellen to tell you her story, and agree to journey together. You can then select “I have a favor to ask”, prompting Sellen to request that you find a founding Glintstone sorcerer named Master Lusat. Sellen will give you the Sellian Sealbreaker, which you can use to enter Sellia Hideaway, just southwest of Fort Faroth in Caelid. The entrance to the cave is hidden opposite some gravestones to the northeast. You can roll into the rock or hit it to reveal the entrance, then make your way through until you find a seal similar to those you encounter at the entrance to the Raya Lucaria Academy. here you can use the Sellian Sealbreaker to find Master Lusat behind the seal, interacting with him to obtain the Stars of Ruin spell.
Founding Rain of Stars
The Founding Rain of Stars sorcery can be found at the Heretical Rise tower, east of the Snow Valley Ruins Overlook Site of Grace in northeastern Mountaintops of the Giants. To reach the tower you’ll need to walk over an invisible bridge, following the white, slow-like patches of white mist which are visible on the surface as they turn left and upwards into the tower. Once inside, turn left and take the elevator up, then the outdoor spiral steps which lead to the highest room, where the chest containing the spell can be found.
Ranni’s Dark Moon
The Ranni’s Dark Moon sorcery can only be acquired by completing Ranni’s questline, which leads you to the Age of the Stars ending. You can then reach the Altar South Site of Grace in southwestern Liurnia of the Lakes. Interact with the statue outside of Chelona’s Rise to trigger “three great wise beasts” to spawn, which you’ll find in the form of giant spirit turtles in the area. One of these can be found on the cliffside west of the tower, and another on a cliff ledge far to the east. Finally, you’ll find one floating in the air above a canopy of trees, far to the north of the tower.
Once all three turtles have been dispatched, you’ll be able to enter the tower and find the spell inside.
Where to Find All Legendary Incantations
The Elden Stars Incantation can be found within Nokron, Eternal City. This region is only accessible after defeating Starscourge Radahn. The entrance to the region is difficult to miss, since it’s an enormous hole in the ground located west of Fort Haight West, in southeastern Limgrave. You’ll see a scattering of rock floating above the hole. Once you’re inside, you’ll need to make your way to the Great Waterfall Basin Site of Grace, then rest in the coffin beneath the waterfall. A cutscene will then begin, at the end of which you’ll find yourself in a new part of the city, at the top of a waterfall far to the north. From here you can head south to activate the Great Waterfall Crest Site of Grace, then turn west and jump onto the large branches while mounted. Turn north onto the branch which leads to a cave filled with ants. Make your way through the cave until you come out at the western side, where you’ll find the Elden Stars Incantation on a tree stump.
Flame of the Fell God
The Flame of the Fell God Incantation is dropped by Adan, Thief of Fire, a boss located at the Malefactor’s Evergaol in southern Liurnia of the Lakes. More specifically, the evergaol is located along the path northwest from the Church of Irith. Once you kill Adan, you’ll loot the Incantation as a reward.
The Greyoll’s Roar Incantation can be purchased after defeating Elder Dragon Greyoll, an enormous dragon boss located near Fort Faroth in southeast Dragonbarrow, Caelid. This dragon has an incredible amount of health, but they are unmoving and they don’t fight back, since they’re near death. However, there are a number of lesser dragons surrounding Greyoll that you’ll need to deal with. Once you’ve defeated Greyoll or all the lesser dragons, you can purchase Greyoll’s Roar from the Cathedral of Dragon Communion, in southwestern Caelid, for 3 Dragon Hearts.
Once you’ve acquired all seven of these spells, you’ll unlock the Legendary Sorceries and Incantations achievement.