Lords and Ladies, the 14th Discworld novel, is set in Lancre during "circle time," at which point the barriers between worlds of the multiverse is particularly thin - a concept that Pratchett has already visited several times in his previous books. In Lancre (home to his well known characters Granny Weatherwax, Nanny Ogg, and Magrat Garlick) the "lords and ladies" (the elves - who should never be referred to by name, as they come when called) are trying to get in. The problem is, most people don't remember the horrors after centuries - they remember how beautiful the Lords and Ladies were. Although even that was false, a glamour, a spell that they weave.
This book borrows the St. Crispin's Day speech from Shakespeare's Henry the Fifth at one point, characteristically mangling it horribly in the name of humour, but is mostly based on Shakespeare's A Midsummer Night's Dream - except that these elves are incredibly vile. "What's the word, starts with 'M'? Elves got none of it." "You mean 'empathy?'" "That's it."
And towards the end he goes with the "temporary change of personality" routine again to get out of a narrative hole, having the normally very meek Magrat go on a totally atypical rampage. He explains it, but not terribly well in my opinion: it seems like a cheap trick to allow him to mess about with a character.
It's okay, but not my favourite Pratchett.