Siemomysł

Siemomysł - (about 900~ to 950~) leader of Polan tribe,duke of Poland, father of Mieszko I.
How we know about him.
In the Gallus Anonymous Chronicle we can read about him as a Duke of Polans before Mieszko I. He is also mentioned in many other chronicles.
What we know about him.
We don't know a lot about him. Perhaps he caused the wkrzanie uprising in germany. He centralized the country a lot, beacause before baptism of Poland, this country was quite centralized as a Pagan country. He probably also conquered Goplans and Mazovians.
Sources.
http://www.staropolska.pl/ang/middleages/Sec_prose/Gall.php3