Millicent quest is one of the longest you’ll find in Elden Ring, which will cover pretty much the entire critical path to see through.
It also involves exploring a secret optional area that will involve the most challenging boss in the entire game, Malenia.
Not only has this redheaded woman got a fascinating and tragic arc, but seeing her quest through to the end will also give you Miquella’s Needle, a key item that can reverse the Lord of Frenzied Flame ending if you had met the Three Fingers.