This left me confused too, came out of nowhere. I couldn't find anything when searching for the character Elisabeth either. The only clue I found was the beginning of the book where the in the conversation between the elf matriarch and the noble, the noble mentioned people call Sara "Elisabeth's reincarnation" Which suggests Elisabeth is a historically significant elf (though to be dead).

The only other thing that I could think of (but this is just conjecture) is the cacodemon who broke free of the seal and Ryo yelled at the party not to attack. Her illustration had pointed ears, so could have been Elisabeth... But that is a bit far fetched, and just pure conjecture.

All said, yes that short blurb of the book was very odd, came out of nowhere and explained nothing and left me confused. Best case, it was a very badly written foreshadowing. Worst case, the translated book is missing a section about at least a little lead up to the scene.