Rellana Twin Moon Knight is an optional Legend Boss introduced in Elden Ring: Shadow of the Erdtree. You don't need to defeat it to complete the game. Rellana is located deep inside the Castle Ensis.

However, she will not sell this set of armor until you’ve defeated Rellana Twin Moon Knight, the boss of the Castle Ensis legacy dungeon—and you may want to grab a few extra Scadutree ...

This is the order I encountered him in. It will come after you’ve beaten Rellana, Twin Moon Knight, or taken the side path around Castle Ensis to skip that fight.) The quickest way to the Church ...

Remembrance of Rellana, the Twin Moon Knight, hewn into the Scadutree. The power of its namesake can be unlocked by the Finger Reader. Alternatively, it can be used to gain a great bounty of runes.