In the Beginning, the Ancient Demon Saint Used Me as a Cauldron 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.nl fastest updating comic site. The Summary is

The first step of cultivation is to make a contract with a demonic familiar. He was untalented and unwanted, until he runs into an ancient demon lord, who happens to take the form of a sexy girl with a tail! She tells him that his foundations are strong and his bloodline is powerful. But this demon is just too alluring, and everyone wants take her away from him. It’s up to him to protect his own demon!