It has been a plot device in several video games and films over the last few decades. There was an eponymous (and quite bad) movie made in 2005. Also Robert Ludlum used it in his novel The Prometheus Deception
So, it is not really that obscure either.
edit: I just found a New Scientist article from 1995 which seems to take the idea somewhat seriously.
we used to widely use a red disinfectant that contains mercury around here in the nineties, so this being called a myth with all the weird mysticism behind had me even more confused
I agree. However, I had heard of "red phosphorus", so the expression "red mercury" sounded vaguely plausible to me.
I suppose that claiming that something is a "myth" is a bullshit excuse journalists use for writing about something that is completely untrue, which they are not normally supposed to do.
Well there are middle Eastern mediaeval writers talking about it, middle eastern sheiks going to Egypt after it, and middle eastern terrorists trying to buy it, so it may not be well known in the west, but is obviously widely believed in certain circles.
Then it may be a good thing that this hoax has been made more well-known like this - expose it for what it is before it becomes a more mainstream scam or dangerous medicine like e.g. the current anti-vax campaign / essential oils trend.
