You'll have to fight Graveward in Borderlands 3 when you reach the vault on Eden-6.

Where do I fight Graveward?

Where To Find Graveward. Since you gotta kill Graveward as part of the main campaign you should already know its location, but just in case, you can find Graveward underneath the Jakobs Estate on Eden-6. Just fast travel to The Floating Tomb and you'll be two seconds away from combat.

What planet is Graveward on?

Graveward is located in the Floating Tomb zone on Eden-6 planet and i show how to get to this boss.