Answer:
Always wax, yet always wane: I melt, succumbed to the flame. Lighting darkness, with fate unblest, I soon devolve to shapeless mess.
Useful tool for who in darkness dwell. Within you, corrupting like a deadly spell.
We travel much, yet prisoners are, and close confined to boot. Yet with any horse, we will keep the pace, and will always go on foot.
What flies without wings?
What surrounds the world, yet dwells within a thimble?
You can always find me in the past. I am created in the present, but the future can not change me. What am I?