The Strongest Guardian Who Devours Gods and Demons is a Manga/Manhwa/Manhua in english language, fantasy 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

In a world where creatures called Demon Spirits once existed. Young Alto is pushed into a well that sealed away a Demon Spirit, due to the schemes of his bullies. To survive, Alto consumes the flesh of the Demon Spirit and becomes a demon himself. The Demon Spirit's name was 'Kamikui' (God Eater) - a unique demon that grew stronger by devouring its opponents' flesh. Having inherited the characteristics of 'Kamikui', Alto grows stronger by consuming monsters. While struggling with his condition, Alto searches for a way to become human again to fulfill his promise with his childhood friend Mei.