There were three realms in this story.
From one of them Humans reside . our first proteginist of this story also called Light is from Human realm . As for profession he is a full fledged Assasin. Our second protagonist is bit odd he is god of this universe, he however incarnate himself as brother of light in same family we called him  Dark .{ Yes his dad prefer harem :⁠-⁠)}. As for Dark he came to enjoy some time . 
Earlier I mentioned three realms one of them is human realm others are Monster and demon realms . 
These worlds connected by great gate called gate of hell . The Human realm gate sealed by our first awakened human & founder of Slur .
The gate that sealed are now going to open. 
In this story we will see how our protagonist becomes awakend and how they protect humanity how they fight these things . 
what is Slur , what are these gates , How one can be awakend, why Dark reincarnated , Powers of awakend , how gate is sealed.
let's enjoy O⁠_⁠o

ASSASSINATION IN ANOTHER WORLD