Kubinashi Seirei-sama ni Kuchizuke wo

From Naono Holics: A deadly curse is slowly killing Theo, and none of the healers or magic users he's ever met have been able to help him - until one day, during his travels, he finds the natural spirit Aluvahani. Aluvahani hates humans and has secrets of his own, not the least of which is that he has lost a considerable part of his powers. Theo has something he wants, and so he binds him with a contract to save himself, promising Theo to break his curse at the end of it, as a reward. But will the two be able to save each other? It might be much harder than either of them thought...