Rage of Harrogath is a Unique Chest Armor in Diablo IV Season 15. It comes from The Butcher (Lair Boss hoard - targeted drop); Duriel (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
Inflicting Damage Over Time on an enemy has up to a [45 - 60]% chance to reduce the Cooldowns of your Skills by 0.5 seconds.
Affixes
Strength
Damage Over Time Multiplier (+12.0 - 60.0%)
Where it drops
The Butcher (Lair Boss hoard - targeted drop)
Duriel (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