A story about Slime, Wisdom and End

A normal boy every night when he goes asleep finds himself in an empty room with only one person named Kray, who tells him stories about a fascinating Slime building a country from the scratch.
But who is the person? Why is she telling stories to the boy? What does she want?
The cover and the "That time I got reincarnated as a slime" are not mine.
Tensura is created by Fuse and I found the cover on google.