This is one of Pratchett's Witches book, the previous being Witches Abroad (1989).
In this book Magrat Garlick, the youngest of the 3, is to be married to King Verence and must learn how to Queen. Meanwhile the Lancre Morris Dancers are preparing for an entertainment for the wedding party and helped by much intake of alcohol (Scumble) they inadvertently awaken the Elves from within the stone circle. Elves are bad. But they have style and glamour which helps to win over the people of Lancre who don't really believe they're harmful. The witches know otherwise and along with Ridcully the slightly incompetent wizzard, who can't spell wizard properly, must set about convincing people about the true nature of said elves and doing something about them...
Utterly silly and typically Pratchett. Good stuff.
The Battle of Forever - A E Van Vogt
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