Heart of the Furnace is a Nightmare Unique relic, hunted from Cinder Wyrm in the Mortal Arena, always at item level 120. It is one of four Exceptional drops on its list — 2.5% of the Uniques that boss gives up, about 1 in 40 — and every kill rolls the list 5 times. It is built on Eternal Cinderwake Relic — step 6 of the Cinderwake Relic line, item level 80 and up.

It always rolls the same numbers: +13% Ultimate Generation, +20 Power, +20% Fire Damage, +22% Burn Effect, +22% Ultimate Generation, +48% Fire Damage, +64% Ultimate Generation, +51% Burn Effect and +12% Power.

One Unique drop in 40 is a Mythical copy: the same item carrying 15–25% Mythical Energy that boosts every stat on it and can never be changed; the 15% floor is 10 times as common as the perfect 25%.

On top of that it carries Heart of the Furnace — Your Ultimate erupts 4 times over at 25% less damage each, every blast setting Burn and Igniting a full Burn, but your maximum health is 12% lower.

It fires Kindling Eye on its own: Every 5th critical strike ignites the target for 80% of the hit.

It leans into Burn.

Wearing it hands you Spirit Burst.

“The Wyrm's first coal, still refusing to be ash.”