The Demon King’s Confession is a Manga/Manhwa/Manhua in english language, Action 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

The ‘curse’ left by the demon king a thousand years ago turns people into magicians. Lapis dreams of becoming a paladin while hiding her identity as a magician. One day, she accidentally summoned the demon king ‘Kara’! The demon king will remove the curse if she helps him, but… Wait, this Demon King’s confession is unusual?! Chasing the stars, Lapis approaches the truth of a thousand years ago.