The Furnace is a Unique Two-Handed Mace (Two-Handed Bludgeoning) in Diablo IV Season 15 for Barbarian, Druid, Necromancer, Paladin, Warlock. It comes from Season 15 Legacy Unique; General Unique loot pool / world drop - not attached to any boss loot table; Horadric Cube: Item Transmutation - Upgrade to Unique from a Common base of the same type.
Lucky Hit: Up to a 40% Chance to Deal +Fire Damage (guaranteed affix) ([1,800 - 4,800])
Where it drops
Season 15 Legacy Unique
General Unique loot pool / world drop - not attached to any boss loot table
Horadric Cube: Item Transmutation - Upgrade to Unique from a Common base of the same type
All Unique items can drop at level 1 in any Difficulty since 3.2.1 (official patch notes)
Possibly also: Uber/Lair boss ladders and Tormented bosses, Helltide chests, Whisper caches
Nightmare Dungeons, Helltide Chests, World Bosses, Kurast Undercity, Purveyor of Curiosities (Murmuring Obols)
Season 15 Legacy Unique: world drop; craftable as Charm (except Leoric's Crown/Messerschmidt's); no specific boss or activity recorded
Can also be crafted into a Unique Charm (Horadric Cube > Craft Unique Charm) per 3.2.1 notes; any Unique Charm has a small chance to drop at Mythic quality.