Pandora gave it to the Gatekeeper as her weapon of choice. Under attack of the Infinite Ones, Gatekeeper gave it back to Pandora, since it has more effect in the hands of Pandora. Pandora kept it afterwards, since Gatekeeper has disappeared through a portal to seal it after her.
The spear transforms into a mystic bolt on being thrown, and detonates in a burst of energy on impact, returning to the thrower in solid form thereafter. Its power is proportional to the strength of the wielder.