Complex sentence: The plan was for Severus to follow his son's army and permanently occupy all of Caladonia, however in 210 Severus became ill and went to York to rest and recover, he kept getting worse until 4 February 211 when he died.
Split into simpler sentences: The plan was for Severus to follow his son's army and permanently occupy all of Caledonia. He kept getting worse until 4 February 211 when he died.