The kryptonite spear was a weapon crafted by Bruce Wayne (while manipulated by Lex Luthor) to take down the superhero Superman. The spear was later utilized by Superman to kill Doomsday at the abandoned port of Gotham City, as kryptonite was literally the only weakness of the seemingly unstoppable monstrosity.
Using the kryptonite he stole from LexCorp, Bruce Wayne forged the spear to kill Superman. He headed to a building near his Bat Signal, sticking it into the ground. When the two of them finally battled later, Batman grabbed the spear, almost impaling him, before being convinced to stop, realizing that they had both been manipulated by Lex Luthor. Lois Lane grabbed the spear, throwing it in to a flooded stairwell. However, they later required it to defeat Doomsday and so she went to retrieve it. Unable to get back up, Superman found and revived her, while diving to retrieve the kryptonite spear.
She rescued him when he was unable to resurface due to being weakened by the spear and threw it away in order for him to regain his strength, before he flew towards Doomsday, grabbing the spear, and stabbing him through with it, killing the creature (but not before Doomsday managed to stab him with a recently formed bone protrusion). After the death of Superman, Batman removed the spear from Doomsday's chest and put it in the Batcave.