Where does “Василии” come from?

Василии (Church Slavonic) comes from Ancient Greek βασίλειος, from Ancient Greek βασιλεύς, from Proto-Hellenic gʷatiléus — chief; king.

Ancestry of “Василии”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	βασίλειος	of the king, kingly, royal
2	Ancient Greek	βασιλεύς	chief, master; king, lord, patron
3	Proto-Hellenic	gʷatiléus	chief; king

Василии — Etymology, Origin & Word History | Etymology Explorer