Fire Knight's Greatsword is a Colossal Sword in Elden Ring. It is a brand new Colossal Sword in the Shadow of the Erdtree DLC. The Fire Knight's Greatsword scales primarily with Strength, Dexterity and Faith and is a good Weapon for Fire Builds.
The Fire Knight's Greatsword weapon can be found at the following location: Drop by Fire Knights (Shadow Keep)
Weapon Skill: Stamp (Upward Cut)
This weapon can be infused and buffed
Fire Knight's Greatsword can be upgraded by using Smithing Stones
Easiest way to farm this is by taking the elevator from the "Storehouse, First Floor" site of grace down and farm the one enemy that wields it.
Despite its flavor text indicating more AR than usual when buffed with fire, this is not the case.
The AR increase referred to in the weapon's description is actually applied to Holy infusions rather than Fire infusions. This is likely a bug.

Attack
Guard
Scaling
Requires
Skills: Stamp (Upward Cut)
Damage Type: Standard/Pierce
FP Cost: FP 5
Weight: Wgt. 16.0
"Greatsword of the Fire Knights. A long, slender blade that undulates like fire. Blessed with a flame incantation. Further flame imbuement will amplify the effect and greatly increase the armament's power. "
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