From beginning till end - this line implies everytime, from the first to the last you see a Zodiac Stone.
in darkness and in light, - this line signifies the evil and the good in the object. It turned the Shrine Knights into Lucavi, but resurrected Malak from the dead.
I wake the sleeping and make them fight. - Raising the dead in battle to fight. Zalera summoned undead to fight in battle with him.
I give the gift of restless slumber and strength of stone, - this line describes the power given to the Lucavi demons. The skill "Nightmare" is used by Queklain and Zalera and "Titan" is used by Velius
to absorb the mighty, but not in flesh nor in bone. - Bloodsuck, which is given to Zalbag with the stones power
All who gaze upon me know it true, - you know its the stone when you see it due to its unique appearance in the game
through darkness and light, for I always do - this is the toughest line...I didn't really give any good hints as to what it is the Zodiac Stone does...you just have to know through paying attention to the game. Every time you see the stone light sparkles from it, even in the darkness of the Deep Dungeon.
Who am I and what do I do? - Zodiac Stone and everytime you see one, it sparkles.