Gae Bulg

The Gáe Bulg (also Gáe Bulga, Gáe Bolg, Gáe Bolga, meaning "notched spear", "belly spear", "bellows-dart," or possibly "lightning spear") was the spear of Cúchulainn in the Ulster Cycle of Irish mythology. It was given to him by his teacher, the warrior princess Scáthach, and its technique was taught only to him.
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[Celtic] "Notched spear". In Old Irish legend, the wonderful spear of Cuchulainn. The notches were created to do the most damage when it was extracted from the body. The spear was made from the bone of a large sea-monster that died in a fight on shore with another of its kind. Cuchulainn received the spear as a gift from Aiofe, the woman-warrior of Alba, and mother of his only son, Connla. With this spear he killed Connla, unbeknownst of the fact that it was his own son. With Gae Bulg he also killed his best friend Ferdiad in the War for the Brown Bull. In the last fight Cuchulainn fought, he hurled the spear at the satirist who demanded it of him; but Cuchulainn died soon after without it.