Kubinashi Seirei-sama ni Kuchizuke wo

A deadly curse is slowly killing Teo, 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 Arvahany.

Arvahany hates humans and has secrets of his own, not the least of which is that he has lost a considerable part of his powers. Teo has something he wants, and so he binds him with a contract to save himself, promising Teo to break his curse at the end of it, as a reward...

(Source: Kadokawa, translated)