Arreat's Bearing is a Unique Pants in Diablo IV Season 15. It drops from Bartuc, Lord of Chaos.
Unique power
Your non-Ultimate Skills that summon Ancients will now summon an additional one. Skills that summon Ancients deal [30.0 - 40.0]% increased damage.
Affixes
Strength
Armor (+183 - 2,450)
Where it drops
Bartuc (Lair Boss hoard - targeted drop)
Any Lair Boss general unique pool / world drop at Torment (chance rises per Torment tier)
Horadric Cube: Item Transmutation - Upgrade to Unique (Common base of same type + 1 Enhanced Primordial Dust + 10 Primordial Dust) rolls a random unique of that item category