I'm dating a Dark Summoner is a Manga/Manhwa/Manhua in english language, SUGGESTIVE series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Amona, a dark summoner and demi-human who uses demonic powers, is the newest addition to the party of the uptight priest Roni. Everyone expected the two to dislike each other and to have nothing to do with each other. But to their surprise, on their first day together as an adventuring party, they had a one-night stand...!