Priory of the Nine Oblivion Guide

Go to the Priory and enter it. Use the symbol on the floor to the left of the entrance, and an entrance to the basement will be revealed (you must have Amiel's ring in your inventory for that). Go even lower, to the Undercroft.

Sir Amiel's ghost will appear and tell you that in order to get Pelina's cuirass, you have to fight all 8 Knights' ghosts. They will engage you into duels one by one. They're not too tough, merely plain warriors who just approach you and hack their way through. When you defeat them all, take the cuirass from the display up ahead.

Now by talking to other Knights you trigger next quests that see you searching for the Crusader's equipment.