The Third Blade is a Unique Sword in Diablo IV Season 15. It comes from Astaroth (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.
Unique power
Your Weapon Mastery Skills are now also Core Skills that have 100% Cooldown Reduction but cost Fury, dealing [140 - 160]% of normal damage.
Affixes
Weapon Damage
+Ranks to Core Skills
Where it drops
Astaroth (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