A fightable, optional boss selectable from the Airship after you have defeated the Dark Aeons in FFX International. Penance is supposedly the "penance" that Yuna and the party pay for betraying Yevon.

http://forums.eyesonff.com/showthread.php?t=80877
http://forums.eyesonff.com/showthread.php?t=80811
http://forums.eyesonff.com/showthread.php?t=80485

Multiple links for related threads on Penance. Maybe we should sticky a thread all about it or something...