Ancients' Oath is a Unique Two-Handed Axe in Diablo IV Season 15. It drops from Andariel (Echo of Andariel).
Unique power
Enemies hit by Steel Grasp take [45 - 60]%[x] increased damage from you for 9 seconds.
Affixes
Weapon Damage
All Damage Multiplier (x% Damage) (3.0 - 10.0%)
Where it drops
Andariel (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