The Sword of Hellfire is one of Agni's unique transcendental skills. The sword can be temporarily summoned by using the Staff of Agni as a medium. It is suited for combat, but its original purpose is letting the user copy any fire-aligned transcendental skill that hits him.
If summoned in Hell, it does not need a medium. In the human realm, a strong medium is needed; if the medium is too weak, it will be destroyed while summoning the sword.
It can only copy skills that failed to instantly kill the user. The learned transcendental skills are far weaker than the originals.
So far we know of three skills Agni copied through the Sword of Hellfire:
- Nine Snakes of Hell (Unique, Owner: Vasuki)
- Extermination (Unique, Owner: Taksaka)
- Fathomless Fire (Unique, Owner: Vritra)
The copied skills can be used only when the Sword of Hellfire is summoned.
It can be said that it's a cheap weapon, considering it even can copy unique fire-aligned transcendental skills.
Agni recently used the Staff of Agni to summon the weapon from the lower levels of Hell in order to end Sagara's attack on Atera. Brahma happened to be in Hell looking for the place the weapon rested at just after it was summoned.