Chapters 1-2 "A Sun in Love's Purgatory"

From Renta!:

Hector is a demon descended from a clan of doctors who made their living researching diseases deep in the bowels of the underworld. As a child, he falls in love with Lord Vet after the demon prince saves him from a group of bullies. A faint trace of that love lingers as he grows to maturity. One day, after being bitten by a stray zombie dog, Lord Vet contracts an incurable disease known as "The Black Scourge". Hector is assigned as Lord Vet's personal physician, but the prince's heart is deeply wounded, and he demands "comfort"...!

Read on Renta!

Chapters 3-6 "Lord of Love's Purgatory"

Mejiro Haizaki's soulful BL fantasy of the demon world!

A priest captures a demon... A life of confinement between a tormenting priest and a gentle demon. And an overly evil priest x an overly pure-hearted demon x a devilish angel...An erotic battle unfolds in a sensual triangle of three! The dark saga of angels and demons begins with this one book. Is the ruler of earth an angel? Or the devil?

On the orders of an angel, Father William captures the 300th son of the devil, Sin, with an angel's specially made collar. The exorcist, called the angel's dog, raises and trains the too-human demon. In captivity, the priest and the demon become attracted to each other...An angel with powerful powers appears and tears them apart!