Quest from: Lathon;
When you're back to the Priory after retrieving all the artifacts, you'll meet a man named Lathon in there. He'll give you Greaves of the Crusader and tell you to get back the Sword of the Crusader. If you want, you can have him follow you on this quest. Go to Underpall Cave. Go through the Cave and the Keep (nothing special in there, just a simple cave filled with undead).
When you're in Underpall Reflecting Chamber, you'll see Sir Roderick's body. In the next room you'll face Lord Berich Vlindrel's ghost. When you defeat him, grab the sword from his body. It is cursed and if you try to equip it you'll, you'll be as well. Tough luck.
Head to Cheydinhal Chapel. It's also being attack by the Aurorans. When you succeed in helping the priests defend their home, touch the altar and the sword will be pure once again. Go back to the Priory and enter the Chapel (it's a smaller building right next to the main one).