Vasavi Shakti

O Sun, Abide to Death. The Anti-Divine Noble Phantasm of Lancer of “Red,” Karna. A god-killing spear given to him by the thunder god Indra in the Mahabharata. The secret weapons among secret weapons, which doesn't exist even in the treasure vault of the King of Heroes. Just as shown in the illustrations within Complete Material IV, his spear transforms and officially becomes this Noble Phantasm after he sheds his golden armor.
It's not like the spear he usually uses is a fake, but it can't activate its name of “Vasavi Shakti: O Sun, Abide to Death” in this state.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia