What poison killed King Hamlet?
In the play, the poison is called "hebenon," and no one knows for sure what that equates to.
In the Second Quarto of 'Hamlet', printed 1604-5, the poison is called "Hebona." The word "Hebenon," as mentioned, is the spelling in the First Folio, 1623. That leaves it unclear exactly which word Shakespeare, himself, used. He might even have used both spellings, at different times, since the play printings are nearly 20 years apart.
Other writers, in the same era, used "heben" or "hebon" for the name of a deadly poison, or for something that was considered especially deadly. Christopher Marlowe, in his play 'The Jew of Malta,' wrote: "... the blood of Hydra, Lerna's bane, The juice of hebon, and Cocytus breath, And all the poisons of the Stygian pool ..."
Shakespeare may have simply gotten the word from reading what Marlowe wrote, but changed it slightly to fit his verse. Marlowe's word "hebon" becomes Shakespeare's word "hebona," as it was printed in the Second Quarto, just by adding an 'a' to the end. Nobody will ever know for sure, but it might be just that easy: Shakespeare read Marlowe's writing, and got the basic word from that. Maybe.