That eye is hideous
Spoiler!
Fck harem, fack harem lovers they are perverts who fantasize about bunch of girls liking them
Its an Outer G**. Well, G** is an understatement since it is an existence that is a literal personification of a part of the Abyss. It is the source of all the mana in that world, and our MC does not know that he using its power. It is also connected to the Five Sins and the second comming of the cleansing.
The Five Sins though monstrous and apocalyptic are not actually malicious entities. They are a kind of natural response to a problem that has occured in the world, and that is "The Black Ones". The "Black Dragon", "Black Wolf", and even the "Black Lambs", that were previously mentioned, are all part of the same bigger picture; and the Sins are the response to that problem.