I explored Hell, where devils were fighting demons (screenshot 1). There were various creatures there, even a huge spider that I had killed before)).
Then, the last guardian of Mephistopheles summoned a horde of demon-spheres, which my summoned slaad could have defeated alone (screenshot 2).
After that, I learned their real names and was able to escape from Hell. Back home, I fought Mephistopheles and killed him (screenshot 3).
He offered Aribeth and Dyken to join him, but the kobold immediately refused, while Aribeth... she initially wanted to, but when I told her that I loved her, she immediately changed her mind)). Later, the story mentioned that we loved each other and lived together for a long time. And after death, we met in Hell, as she had said.
P.S. I was wrong about the illusion mirrors; they became the coolest units in the Dungeon).