Elden Ring: Where to Find Master Lusat (Quest by Sellen)
Master Lusat, the former mentor of the sorcerer Theron, is locked in a hidden crystal cavern near the sorcerous city of Celia in Kelyid.

Once the Tarnished have acquired Comet Azur from the original wizard Azur on Mt. Gelmir, they can return to Sorceress Sellen in the Waypoint Ruins for the first quest of her questline - finding Master Lusat in the Elden Ring. She will gift the protagonist with a Sellian Sealbreaker, a key item needed to find her former mentor. The Sealbreaker name should give players an idea of where they should start their search, specifically Sellia, the city of wizardry.
Sellia is located near the center of Caelid in the northeast of the Aeonia Swamp. If players haven't explored the Kelid area yet, they can use the Teleport Traps Chest in the Dragonburn Ruins to quickly get to the City of Sorcerer. This chest will transport the Tarnished to the Sellia Crystal Tunnel relatively northwest of town. However, Master Lusat is not actually in Sellia, but is locked away in the Sellia Hideaway, a mine accessible through an illusory portal atop a hill east of the enchanted town.
Master Lusat's Location in Elden Ring

To enter the Sellia Hideaway and find Lusat's location on the Elden Ring, the player must follow the trail starting at the Sellia Backstreets Site of Grace. Watch out for Elden Ring's rolling ball traps and undead skeletons that spawn along the way. Once at the Plague Chapel, head north to the cemetery and tap the rock behind the center of the cemetery to reveal the hidden cave entrance.
It is highly recommended that the Tarnished bring a lantern, flashlight, or lighting spell into Sellia Hideaway, as the tunnels are extremely dark. Another illusory wall will block the path, but players may be aware of this as nearby crystal-mining enemies can be a little noisy. The first large area Tarnished will encounter has a pit with grafted scions running deep within it. Those who want to fight the demon and loot a few items can drop to do so, but the pit where Master Lusat is held in the Elden Ring will be the following area.
The player can identify which alcove has a hostile snail on it by the long crystal walkway. Jump down but watch out for the Raya Lucaria Sorcerer guarding the spot below. After killing the wizard, the Tarnished will find a small tunnel sealed off by a bright, glowing guard. Use Sellen's Sellian Sealbreaker to remove barriers and Follow this path down to find Master Lusat at the end.
Rewards for Finding Lustat's Location

Players can attempt to interact with Master Lustat in the Elden Ring, but they will only receive the Destroyer Star spell and then be completely ignored. However, the Doom Star isn't the only perk Lustat can offer players. In order to get his armor, the Tarnished should return to the Lustat Master's place in the Elden Ring after fully completing Sellen's questline, at which point they'll find Lustat's suit, including: