Ultima Weapon?
Find me by land, or much later by sky
When doom is at hand, the first option will die.(not sure about land, but the sword can only be gotten by fighting the WEAPON by air)
Twice I'm there when you've escaped via rope;
Once while unconscious and regaining hope. (you have to fight WEAPON twice in the air at least and once while Cloud's unconscious)
It should be noted, I've more than one form--
Sometimes I'm chilling, but usually I'm warm. (two forms, the WEAPON and the sword)
If you are dextrous, then carry me well (you must be skilled to use it?)
To coax divine pow'r from an old corpse's shell (you get the Ultima Weapon by killing Ultima WEAPON)