Sabre of Tsasgal is a Unique Sword in Diablo IV Season 15. It comes from Duriel (Lair Boss hoard - targeted drop); Harbinger of Hatred (also listed in one guide's Lair Boss loot table, Season 14 data); Any Lair Boss general unique pool / world drop at Torment (chance rises per Torment tier).
Unique power
You gain [3.0 - 4.0]%[x] increased damage for each damaging Skill you have equipped, and an additional [6.0 - 8.0]%[x] for 15 seconds when each of those Skills are used.
Affixes
Weapon Damage
Cooldown Reduction (+3.0 - 8.0%)
Where it drops
Duriel (Lair Boss hoard - targeted drop)
Harbinger of Hatred (also listed in one guide's Lair Boss loot table, Season 14 data)
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