Phantasmal Greatsword, Felling of the Sky Demon. The Noble Phantasm of Saber of “Black,” Siegfried. The Nibelungs, a tribe which lived in an underground country, requested that Siegfried fairly and impartially divide up and distribute their treasure, and he accepted the request. Balmung was planned to be given to him as payment for this. However, two kings felt that the allocation of treasure was done unfairly (both kings felt they were being treated unfairly, so it's highly possible that it was actually divided up fairly without Siegfried favoring either one) and fought Siegfried, but he killed them in return.
The remaining treasure which no longer had an owner became Siegfried's, and Balmung also became his weapon. He wielded Balmung in every battle after that. It is also said that he won in the battle against Fafnir thanks to Balmung.
It is a sword of twilight that has reached A rank and possesses the quality of both a holy and cursed sword. Perhaps because it was owned by a king, it specializes in fighting against armies, shooting off a semi-circular slash wave. The blue jewel attached to the handle stores True Ether to use as a boost when activating the Noble Phantasm.
Now then, it does have a fast activation rate for a so-called beam weapon, but in a comparison of speed, Siegfried when he was alive was the fastest in activating it, with Sieg being the next fastest and the Servant Siegfried being the slowest out of them.
Sieg possesses the “Galvanism” ability, which the Servant Siegfried does not have, and can draw out the needed mana all at once when activating the Noble Phantasm. The living Siegfried, on the other hand, had his inborn disposition combined with the transformation of his heart due to being bathed in and drinking a dragon's blood, so he could draw forth mana and fire the slash wave even faster than Sieg. Unfortunately, the moment he was summoned as the Servant Saber, that special characteristic of his was highly degraded.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia