Elden Ring’s Malenia is an optional encounter but also by far the hardest boss in the game.
She is however a required opponent if you wish to complete Millicent’s questline and obtain Miquella’s Needle, which is a key item for reversing the Lord of Frenzied Flame ending if you had visited the Three Fingers.
As Malenia has attacks that can easily one-shot you, it’s highly recommended using spirit ash summons for this fight just to give you some breathing room.