I'm on the Cossack storyline and have succeeded in putting Radizall on the throne but I am not getting an conversation option from him about the 'Black Mace'.
The last one I just completed was Radizall's Rebellion.
I'm stuck with a quest called "Seizure of Kiev" (which was given by Radizall) even though Poland owns Kiev. I don't know if that's the cause for the glitch as I recall; when Radizall joined my party, that I lost my options in discussing the Black Mace.
After capturing Kiev the order changes from Capture Kiev to
"A new order has come from Janusz Radiwill -- capture Janusz Radziwill"
Kiev mayor has no info about mace, Radziwill does not have further options either.