You shall carry your sibling, be his support and guide.
He shall be your shelter, guardian, aegis firm.
United you stand, divided you fall.
Brendan stared at the creature in the pale moonlight, transfixed by the silvery petals that glistened beneath that perfect purity. It was not until his final breath that he gathered his wits enough to scream after the thing had descended on him, all muscle and thorns and hard, barklike skin. It was too impossible, even the crimson blossoms that shone in the light of the moon.
When you sleep, your soul briefly returns to the place where it was created/goes to when it dies, and seeks nourishment. Everybody's soul is created with a little link (or you could say a greater entity calls it back) to that place so that it can return easily, but upon death, the soul returns via that and annihilates the link as if returns.
Also, as an added thingy, you could say the entity supplying the link, or residue of the links themselves, (whichever) is the source of all arcane power.