There are actually several operas, though Verdi's won the test of time. I mean, if a King - one famous for his scandals, his opera love and opera financing, no less - gets assassinated - well, shot at - in the opera during a masque ball, you don't expect composers to resist, do you? Poor old Verdi and his librettists had to change their script several times, though, as by the 19th century censorship was very tetchy about the depiction of assassinations involving royalty. So Gustav first got demoted from Swedish King to Duke of Pomerania, and then, when censorship still wasn't satisfied and demanded the entire action to be transferred to another continent, he ended up as "Riccardo, Governor in Boston". Bet you didn't know the early Puritans partied like that, did you?

(These days, the opera is produced in both the rewritten and the original draft version.)

(This problem kept happening to Verdi. It's also why Rigoletto, which is based on Victor Hugo's novella Le Roi S'Amuse, got transfered to Mantua, with King Francis I. ending up as the anomymous Duke of Mantua. (Who doesn't die, but there is an assassination attempt. Like I said - tetchy censorship was tetchy.)