Shadow of the Moon (yunee)

Yeon-Ho is a military officer who uses his supernatural abilities to protect the people from various demon-like beasts. Although the abilities he possesses are extraordinary, his energy gets thrown into a frenzy upon exceedingly long usage, and he loses consciousness if he isn't purified right after subduing those ferocious beasts.

Yet, upon opening his eyes the next morning, he realizes that his frenzied energy had unbelievably disappeared after having sexual intercourse with someone while unconscious!

Having suspicions that the Purifier, Hae-Jin, who has only ever been overtly concerned about his clan's authority and influence, couldn't have possibly purified him, Yeon-Ho pretends to lose consciousness once again... Only to grab hold of Hae-Ryung, the Purifier who enters his room for purification, and who is clearly a stranger but possesses an oddly familiar aura.

"Who... are you?"

(Source: MrBlue, translated)

Note: Includes two extra chapters.