The Betrayer gives the Celestial Locator to the Doom Slayer.
The Celestial Locator is a orb-shaped device that can be used to locate Hell Priests. It is formed by two comonents: a "lantern" with inside a golden sphere and a blue glowing sphere, in possesion of Valen (he had two more). Once assembled, the markings on the golden sphere will glow blue.