The Master Bolt was a mythical weapon and the symbol of power of the King of the Greek Olympian Gods: Zeus, a spear that used immense magic channeled through it to create the Lycanthrope Curse twice: once in myth by Odin to create the Monster God Fenrir, and a second time by Zeus himself to turn all of House Arcadia into Wolves.
Originally, it was the Spear known as Gungnir wielded in ancient myth by the Norse God Odin, the Spear was lost however when Fenrir ate the God during Ragnarok but was also slain shortly afterwards.
At some point it came into the possession of the Greek God Zeus following the apparent recreation of the world following Ragnarok, Zeus used it in a ten year war with his father and his family: the Titans, Zeus used its power to slice off the peak of Mount Othrys where the Titans made their home.
Following its usage by Zeus, it was entrusted to Arcus whom intended to hunt down the Original Werewolves and all their Immortal Sires, at one point a descendant of Arcus whom would become named Saint Longinus pierced the side of Jesus Christ with the Bolt believing him to be of House Arcadia and it became renamed once again as the Spear of Longinus.
By the modern era it is revealed that without a Pagan God to empower it, the Bolt hasn't been recharged in quite some time, and that it only has one more charge left - with only enough charge remaining to kill only one of the four surviving members of House Arcadia - unless it misses its target where its charge would then be wasted.