A divine Gift that Kuro Usagi received from one of her race's 12 patron gods. It shines with the light of the sun and is said to make the wearer immortal.
The light it emits is exactly the same as the Sun and was effective against Black Death that was casted by Percher . It is also said to grant the wearer immortality.
However, he was later tricked by the king of the gods, Indra, into giving up the armor to him, and was granted his spear as compensation. This might also be why the Sun Armor and Spear of Indra can not be used simultaneously, as neither Indra nor Karna wielded the two items at the same time.