How was Fridlevus II related to Halfdan?
Fridlevus II Fridleif (Ynglinga saga) was a son of Fróði of the Skjöldung (Scylding) lineage. In that work he was the brother of Halfdan and the father of Áli the Strong.
John, Prince of Schleswig-Holstein Johan of Schleswig-Holstein (9 July 1583 – 28 October 1602) was the youngest son of Frederick II of Denmark and Norway and Sophia of Mecklenburg-Schwerin. He went to Russia in 1602 as the bridegroom of Boris Godunov's daughter Ksenia (Xenia), but fell ill and died before the marriage could take place.