First you must get Legendary Sorcery  “Comet Azur”. Then go talk to Sorceress Sellen in Waypoint Ruins. Pick her dialogues “Show Azur’s Sorcery” > “Tell Me Your Story” > “Let Us Journey Together” > “I Have a Favor to Ask”. Then she gives you the “Sellian Sealbreaker” item. Now head to the north of Caelid: Dragonbarrow region. Drop down the cliffs south of where you defeated Greyoll the big sleeping dragon, then ride alongside the edge of the walls. One of the walls with a sorcerer enemy a few meters in front of it is a trick wall that disappears when you touch it or attack it, leading you into the Sellia Hideaway tunnel. Get to the end of the tunnel, drop down from the purple crystals to find a blue magic seal door gate at the bottom that you can now open with the “Sellian Sealbreaker”. Behind this you find an NPC, interact with it and he will give you the spell.