Koori no Mamono no Monogatari

From Storm in Heaven:

Deep in a cave lies a demon trapped in ice who will eat any human who dares enter. The demon, Blood, waits for the human who will cry the purest tears, for the gem formed from those tears will break the spell that binds him. Then one day, a young man named Ishuca appears in the cave - and knowing that his weak heart will not last long, he has come to die. Touched by Ishuca's gentle nature, Blood gradually begins to change from the cruel, ice-cold demon he once was.