The lance is mentioned in the Gospel of John. To make sure that Jesus was dead on the cross, a Roman soldier (in later texts called Longinus) stabbed him in the side.
It is often identified with Balin as the weapon used to make the Dolorous Stroke. Richard Wagner directly identifies the Spear of Longinius with the spear that wounded the Fisher King in his opera Parsifal.
Melora, daughter of Arthur, is tasked with recovering the Spear from Africa.
In modern times, the legend of the Spear has been linked to many occult and New Age conspiracies.