The Last Battle describes the end of the world of Narnia. Jill and Eustace return to rescue Narnia from the ape Shift - who tricks a donkey into impersonating Aslan and thus precipitating a showdown between the Calormenes and King Tirian.
There is a final scene like Judgment Day (Lewis's everlasting usurpation of Christian ethics, sigh). Aslan divides all animals in the loyal ones and in those who have lost faith. The latter become ordinary animals and vanish into an unmentioned place. The believers follow Aslan into Aslan's Country, the "Real Narnia" and perceive that this was only the beginning of the true story, "which goes on forever, and in which every chapter is better than the one before."

took it to the meet-up as part of "conquer Mt. TBR"

Looking for Harry Potter volumes I stumbled upon almost the whole collection of the Chronicles of Narnia. I decided to take them all and keep them together. I'll have to look for the missing one, though.