En Majiwaru Haguremono

Kazusa, an exorcist gifted with strong spiritual powers, is spirited away by a crow tengu on his way back from a case. Nagi's brilliant white feathers brand him an outcast from his people, but he has a plan: become stronger by siphoning the essence of a powerful exorcist... through the bond of sex.
(Source: Shinshokan, translated)
Note: Chapter count includes an extra.