This item is a reference to the spear that pierced the side of Jesus as He hung on the cross (also known as the Spear of Longinus), according to the Gospel/Scripture of John.
This item is used in The Guardian (challenge #30), where it also has a completely unique sprite that is not used anywhere else in the game. The spear is reskinned to look like a sword, which is purely visual and does not change its effects, however.
Bugs
Bug!
If the Spear Of Destiny is removed from the player's items (for example, when using the D4), the spear will stay on in the level in the position where it was before the item was lost until Isaac moves to another room. This is purely a visual bug.
Bug!
When Pause is used with Spear Of Destiny it creates a second spear that stays in place where Isaac used "Pause" which lasts for the active item's duration. Moreover, both spears are flashing and can not interact with enemies for the duration.
Bug!
Swapping characters using Esau Jr. or as Tainted Lazarus will leave a stationary copy of the spear until Isaac leaves the room. The stationary spear is only visual, lacking collision.