If derivatives of Slav are not used then it's more likely there's an official name like "Union of the Kingdoms of" with a common unofficial name like "Union of CityX" or "The Dynasty" or "the Ntuple Crown".
Especially since there's no geographic feature that crosses the kingdoms.
Another name could be "Crown of St Y" with a saint adopted as patron of the kingdoms in union.
Especially since there's no geographic feature that crosses the kingdoms.
Another name could be "Crown of St Y" with a saint adopted as patron of the kingdoms in union.