The simple past tense of the word to slay is slew. The past participle is slain. The word itself is archaic, and I suspect it’s in our colloquial vocabulary only because of fantasy novels and role-playing games, in which slaying is so much more fantastical than mere killing. For some reason, I keep coming across writing by people who don’t know how to conjugate to slay, usually on Wikipedia, where most of the writers appear to be high-school students. But here’s a college professor:
‘We’ve now seen that you can package the complexity of a large brain in a tiny packet,’ said Lee Berger, a paleoanthropologist at Wits University in South Africa and an author of the paper published in the journal Proceedings of the National Academy of Sciences. ‘Almost in one fell swoop we slayed the sacred cow that complexity in the hominid brain was directly associated with increasing brain size.’
Nicholas St. Fleur, “Tiny Brains of Extinct Human Relative Had Complex Features”, New York Times, 14 May 2018: https://www.nytimes.com/2018/05/14/science/homo-naledi-brain.html
(“Wits University”, by the way, is the University of the Witwatersrand. A bit like calling the University of Connecticut “Yukon” in the Newspaper of Record, don’t you think?)
Wikipedia says Prof. Berger is American-born, so I presume he’s a native speaker of English and has no excuse. Maybe he was misquoted. I doubt it.