If you want to learn more about Sellen, especially in your prep for the upcoming Elden Ring Shadow of the Erd Tree DLC, here's how to complete Sorceress Sellen's sidequest.

One of the Ancient Dragon Smithing Stones that is a quest reward, you can get this one at the end of Sorceress Sellen's Questline - but it will require you to betray her in a final fight.