Konoyo Ibun

In the immediate family of the story's protagonist everyone seems to die early, and other relatives talk about a curse. When soon after his grandfather's death he comes down with an incurable disease, and is overwhelmed by grief and loneliness, in desperation he remembers his grandfather's last words -- that somebody was watching over them. He can only find an odd fossil tooth where his grandfather had pointed, but when he picks it up and accidentally gets some of his blood on it, that summons a guardian spirit, who promises to cure him. Maybe he should be pleased, but said guardian has some strange methods for healing.