The update that brought event #6 (~2 months after launch) added Spartacus and Achilles (first bow using souls). Not sure when Hercules was added, but looking at the comments, the wiki page for her wasn't created until mid-September (~5.5 months after launch).
The update that brought event #13 (1 week short of 5 months after launch) seems to be the one that introduced Aleister Crowley and Alessandro di Cagliostro (first arcane users). Morgan le Fay's wiki page got its first comment in early November (little over 7 months after launch).
I don't see any event-only heroic souls.