Luin (also known as Luin Celtchar or Dubthach's Spear)
In the Ulster cycle of Irish mythology, Luin is the famously bloodthirsty spear of Celtchar. It is said that the spear had to be continually dipped in a cauldron of black poison or blood in order to contain it's power. Otherwise it would burst into flames and consume the wielder and possibly burn down his place of residence. When Celtchar was tasked with slaying his own dog Doelchu because it had grown large and uncontrollable, he did so reluctantly using Luin. After the deed was done, he raised the spear in the air and a drop of the dog's poisonous blood slid down the shaft of the spear and killed him.
According to The Destruction of Da Derga's Hostel, the spear was later found by Dubthach Doel Ulad at the battle of Mag Tured.