Source: finalfantasythings
The Warrior of Light's biography in the Museum in Dissidia 012 Final Fantasy states that "after the final cycle concludes, he arises in a foreign land...the Warrior of Light sets out towards the castle with a darkened crystal in hand."
In the prologue to the first Final Fantasy, "four warriors arrive, each holding an [blackened] orb."
Source: finalfantasythings