Sir Caius was a member of the old Knights of the Nine. He was killed by Sir Berich while trying to stop him from leaving to go fight in the War of the Red Diamond and take the Sword and Greaves of the Crusader.
Caius blames himself for Berich becoming what he turned into, saying that if he hadn't drawn his weapon and fought his friend none of it would have happened. Despite this, he had hoped to reconcile with his friend in the after life, something that has yet to happen as his spirit never arrived to guard the last relic.
Sir Caius is first encountered by the Hero while he is searching the Priory of the Nine for the Cuirass of the Crusader. Sir Caius, as well the the rest of the knights spirits (excluding Berich) are in the under-croft guarding there only remaining relic they have. Sir Caius and the rest of the knights will each fight the Hero to see if he is worthy to take the Cuirass and be told where to find the rest of the relics.
The Hero has discovered that the Knights of the Nine had a Priory in the West Weald and that one of the relics may still be there.