The Cursed Tomb Treasure side quest in Hogwarts Legacy can be obtained by heading to Manor Glen at Manor Cape near the West Manor Cape Floo Flame. Enter the gated manor then head inside the ruined house to find a set of stairs leading down to the basement.
Use Alohomora Level II to unlock the door then head inside and work your way through the basement fighting Inferi along the way. When you reach the big open room, you’ll have to fight off a wave of Inferi then the Lord of the Manor shows up. Take care of him then head into the back room. Ailsa Travers will then appear and you’ll need to defeat her too. Examine the body next to the chest to find a note from Photine then use Alohomora Level I to open the chest.
You’ll receive a map officially starting the quest. Travel to the Tomb of Treachery Floo Flame in the northern part of Poidsear Coast, which can be found behind a waterfall. Head inside the tomb then follow the path north. Take the second left to find a room with a collection chest in it.
When you reach the coffin, collect the note on it then continue heading north using a fire spell to burn the web blocking your path. This will bring you to a sealed door that requires three moths to open. They can be found in the two adjacent rooms. Bring them to the door using Lumos to open it then head up the stairs. Defeat the Inferni then look at the map you received in the manor.
You’ll need to use Flipendo on the tiles to make them match the map. This will cause a chest to rise out of the ground. Loot it to receive the Treasure-Seeker’s Attire appearance and complete the quest.