Art not by me but found on Pinterest.

Gateway to hell or simply GTH, is an organization that specialises in destroying creatures born from the dark called Cizers. Founded by General Harold Cromwell, a group of talented Helix Slayers make a home there.

Shuu Miyazaki is a 17 year old who had awakened his slayer powers. Unable to control it, he turns to GTH for help. He then begins his Cizer Slaying journey. However, he and the organization have a powerful foe running around. Amsiel Zopha or better known as Satan himself. A being that wants to end humam life and bring about a new era.

Shuu and the others have to stop him from completing his plans or else, humanity will fall into the hands of darkness.