Mikołaj z Radomia
1400-1450
This composer of early polyphony from Poland is a genuine mystery. He is assumed (because of his name) to have come from the Polish town of Radom south of Warsaw. It is believed that he was attached to the court of Władysław II Jagiełło although there’s little solid evidence to support this.
However Władysław was Duke of Lithuania but became King of Poland in 1386. So a talented composer of liturgical pieces might well be associated with the royal court.
As to the rest, the most surprising thing is, perhaps only to non Polish people, is the extraordinary number of people called Mikołaj z Radomia or a variant of that name - who might be our composer. Early 15th century folks from Radom called Mikołaj or Nicholas were not rare.
So a genuine mystery, but someone with that name composed some rather beautiful polyphony a few pieces of which have survived. This piece below is a good example:

