Hellbrand Signet is a Unique Ring in Diablo IV Season 15. It comes from The Butcher (Lair Boss loot table); Butcher (Lair Boss loot table, boosted class-unique drop).
Unique power
Sigil of Chaos gains the Diabolic Rebuke Variant for free and deals [100 - 130]%[x] increased damage.
Affixes
Willpower
Skill Ranks to Sigil of Chaos
Where it drops
The Butcher (Lair Boss loot table)
Butcher (Lair Boss loot table, boosted class-unique drop)