Arsenic is called the "King of Poisons" and the "Poison of Kings" due to its long history. Interested readers (I'm looking at you, Jason) can GIY or perhaps even suggest a reference in the comments.
Lesser known is arsenic's more recent role in our own poison gas warfare program. I mentioned Lewisite here, but I thought that a more formal introduction was in order. Even a non chemist can recognize arsenic's central role in Lewisite's structure. Here it is:
|Lewisite, chemical structure|
We made a lot of Lewisite shortly after WW I but we never deployed it as far as I know. Ridding our stockpiles has taken forever and an old cache was recently found in Washington, D.C. Arsenic just can't doff its nasty reputation, even cloaked in disguise.