In Diablo 4, the final challenge for players is to face off against the Echo of Lilith, the boss of the capstone dungeon in World Tier IV. This encounter should only be undertaken once your character has reached an endgame level of power and has crafted reliably Masterworked gear. Mythic Unique items are not required to defeat her but will make the entire boss fight proceed much faster and smoother, especially items like Tyrael’s Might and the Harlequin Crest Unique.

https://screenrant.com/diablo-4-how-to-beat-echo-lilith-season-5/