Doombringer is a Mythic Unique One-Handed Sword in Diablo IV Season 15 for All classes. It drops from Duriel, King of Maggots, Andariel (Echo of Andariel), Harbinger of Hatred, The Butcher, Belial, Lord of Lies, Astaroth, Bartuc, Lord of Chaos.
Unique power
Lucky Hit: Up to a 40% chance to deal [item-power scaled] Shadow damage to surrounding enemies and reduce their damage dealt by 25% for 5 seconds.
Affixes
+Weapon Damage ([94.2 - 157])
+% Maximum Life ([6 - 8]%)
Where it drops
Craft: 2 Resplendent Sparks
Mythic pool: any Tormented Lair Boss, Torment 1+
World drop (any difficulty, drop rate scales with Torment tier)
Horadric Cube: Upgrade Direct Item to Mythic ('Crafted' tag) - S15 recipe
Season Rank rewards: 5 Mythic Unique Caches + up to 10 Resplendent Sparks
Jeweler Rune Crafting produces a random Mythic of the slot (no Crafted tag)
Horadric Cube: craft with Resplendent Sparks / Runes; S15: Upgrade Direct Item to Mythic ('Crafted' tag)
Echo of Mephisto: 3.2.1 'Increased overall loot quality for Echo of Mephisto, including a guaranteed Mythic Drop'.