Lullaby of the Dawn, Volume 1 (manga)

Elva is a battle shaman whose life is eroded bit by bit every time he drives back monsters that emerge from the black sea. Indignant when he sees this, a young boy named Alto is determined to find a cure and stay by Elva's side. After 8 long years, Alto has become a fearless young man, though he hides his love deep inside.

And despite only expecting to live for a few more years, Elva seems to be healing as he spends time by Alto's side.