Once the light of all four ORBS is restored, you are ready for the end of your quest. Before you depart, there are just two loose ends left to tie up.
Xcalber
If you picked up the ADAMANT in the Floating Castle, you should take it to the Dwarf Cave and speak to Smith the Blacksmith. He will take it and forge Xcalber, the best weapon in the game other than the Masmune. Only a KNIGHT can use it, and it will increase his power considerably.
What's truly surprising is that the Xcalber is actually supposed to be significantly more powerful than it actually is. It is intended to have an affinity with all eight elements, and be especially effective against all eight monster types. In effect, the Xcalber is supposed to have a boost to both Damage and Hit Rate against almost every monster in the game. Even without this, it's a fantastic weapon.
Return to Crescent Lake
The Circle of Sages in Crescent Lake can often be relied upon to give you updates on your quest, and it is they that will reveal the existence of the time warp in the Temple of Fiends once all four ORBS have had their light restored. They will also explain the idea of the time loop, as much as it actually can be explained. You don't need to come here before finishing the game, but it should clarify what it is you're actually trying to accomplish.
The Sky Warriors
Last but not least, you can now speak to the bats in the central room of the Temple of Fiends. It turns out that these are the Sky Warriors of Lefein, who were turned into bats. They've regained the power of speech, and can explain more about the Fiends and the nature of your quest.