You could go for a nice bit of immortality. WE KNEW YOU WOULD and the others would know too So do as I say, will you?
You hold up the Book and after giving it one last wistful look, pitch it straight into the Gate - it flips open and is blown on the stormy winds within, spiraling on out of sight as bizarre entities from the further reaches of demonhood take brief notice. NOW THE WORDS the words the words
You open your mouth, and let the demons speak for you. The crevice you currently occupy folds back in, the corroding influence of the Gate withdrawing quickly, and the hole between planes seizes up and covers itself with metaphysical scar tissue, a thickening of ineffable spacetime. You are pulled backward, not by the words, but by the demons helping you say them, and the stonework snaps shut like a jaw as you find yourself in...
... the laboratory of the Vizier, disorderly and ransacked for the mad rituals of the presence that displaced his mind, the notes all abruptly stopping when he ran out of space on the walls to keep writing them. All is silent, and it takes you a moment to realize that it is also completely dark, as you still appear to be able to see perfectly. Consider this a gift in advance.
You step back through the door and, in the hallway leading through the ancient catacombs beneath the palace, find your operatives still keeping together, cowering for fear of being taken by creatures in the dark. They seem only mildly reassured by your presence as you order them to follow you. And they only grow increasingly disturbed as light is eventually shed on you - arms covered in blood up to your shoulders, your clothes a hastily-assembled shambles, your face still caked in gore with bits of sinew stuck in your teeth, the membranous sack of soggy humanoid body parts slung over your shoulder and most of all your eyes, still profoundly wrong.
Was this a success? A failure? Whatever it was, you'll need to explain this to the Queen. She eagerly awaits you at the top of the steps, having been called as soon as the servants could confirm that it is indeed your crew coming back. It has been perhaps an hour since you went down.
Pick 3:
A) Claim you saved the world. NOBODY CAN PROVE THAT YOU DIDN'T
B) Boldly lie about the Book. discretion is the best policy
C) Expound in excruciating detail about the Vizier. She will not want to know more than that.
D) Confuse her with the finer points of planar metaphysics. things mortal minds are far too stupid to comprehend
E) Break down in tears at the horrors you saw. The mad prophets knew how to avoid questioning at least.
F) Offhandedly mention the apprentice you seduced and then ate. SHOW DOMINANCE
G) Say that you've seen things these people wouldn't believe. People will believe anything, of course, unless you tell them not to.
H) Vomit profusely at the Queen's feet. THE UNBEATABLE EXIT STRATEGY
I) Kiss the Queen. save the world get the girl