The novel "Cards on the Table" (1936) features a crossover between four recurring characters of Agatha Christie. 1) Hercule Poirot was introduced in "The Mysterious Affair at Styles" (1920) and was Christie's most frequently appearing protagonist. 2) Colonel Race, a secret agent of the MI5, was introduced in the novel "The Man in the Brown Suit" (1924). 3) Superintendent Battle of the Scotland Yard was previously among the main characters in the novels "The Secret of Chimneys" (1925) and "The Seven Dials Mystery" (1929). 4) Ariadne Oliver was introduced in the short story "The Case of the Discontented Soldier" (1932) as a freelance assistant to protagonist Parker Pyne. She had also appeared in the short story "The Case of the Rich Woman" (1932). Both of the stories were published in book form in the collection "Parker Pyne Investigates" (1934).
The surname 'Shaitana' means 'the naughty one' or 'the devil' in Hindi, which in turn comes from Arabic. "Shaitana" is a the feminine gender of the masculine noun "Shaitan" in Arabic, which likewise means devil. (There's no distinction between masculine and feminine in English nouns. bear that in mind.)
Ariadne Oliver, played by Zöe Wanamaker, says to Rhoda Dawes, played by Honeysuckle Weeks, that one can't trust a person from Wales. Miss Weeks was born in Wales.