Souleater

Surtur's giant sword. Made out of obsidian. Originally enchanted with gnomish runes in order to absorb the spirit of those slain with it, it appears that either this enchantment has been lost, or the effect requires an extra component that Surtur lacks.

It is the source of Surtur's aberrant personality, as it contains the souls of Flame the crazed sorcerer and the silent but wrathful Hero.

Like anything made out of obsidian, it's essentially indestructible, and by extension doesn't need sharpening. Due to being linked to Surtur's physionomy, it can change shape and size according to his mental state. The spirit of Flame trapped inside the sword essentially acts as a fire enchantment.

The Souleater's seemingly inconsistent effects may be the consequence of faulty rune usage and application on its non-gnomish creators' part.

Trivia

While the sword bears a strong resemblance to the weapon used by Guts in the Berserk franchise, it was actually inspired the Buster Sword from Final Fantasy VII, as the author had come up with the idea before being aware of Berserk.