Charles IX, also Carl, reigned as King of Sweden from 1604 until his death. He was the youngest son of King Gustav I and of his second wife, Margaret Leijonhufvud, the brother of King Eric XIV and of King John III, and the uncle of Sigismund, who became king both of Sweden and of Poland. By his father's will Charles received, by way of appanage, the Duchy of Södermanland, which included the provinces of Närke and Värmland; but he did not come into actual possession of them till after the fall of Eric and the succession to the throne of John in 1568.
Both Charles and one of his predecessors, Eric XIV, took their regnal numbers according to a fictitious history of Sweden. He was actually the third Swedish king called Charles.
He came into the throne by championing the Protestant cause during the increasingly tense times of religious strife between competing sects of Christianity. Just over a decade after his death, these would re-ignite in the Thirty Years' War of 1618-1648. These conflicts had already caused the dynastic squabble rooted in religious freedom that deposed Charles' nephew and brought Charles to rule as king of Sweden.