From the introduction:
In this world, there exist beings that defy human comprehension. Gods, demons, and many more from mighty dragons to simple spirits. Every tale has a grain of truth, and there are many tales. But the details of this world are sparse, because they simply don’t matter.
There exists a young child. His parents died when he was five, and his relatives have passed him around, speaking badly of him behind his back and sometimes not so secretly. Eventually, he landed in the hands of his uncle. It should say something about the others that simply ignoring him as the uncle did was likely the kindest welcome the child had received.
However, the uncle soon had some form of mental breakdown, and the child was left alone. At this point, he did the one thing the uncle asked him not to do, and entered the old shed… and found the secret passageway into its basement.
And accidentally summoned an outer god and promptly asked her to act as his elder sister. Don’t ask me how he got away with it. Regardless, the two of them now live together in relative peace. And when it comes time to pay her price… Well. She is the mother of a thousand young.