Ya,That is a Boss,but it is an OPTIONAL one :)
You can unlock it after you had defeated ALL the Dark Aeons...lol..
Most of the player would find it difficul to do it...
especially killing Dark Yojimbo.....
you had to defeat it directly 5 times without ANY rest.....it's kinda hard
for me too.....but i did it at last :)
Penence is kinda like Sin.You need to defeat it part by part. :)
Good luck for you to defeat the Dark Aeon first. :)
PS:If i'm not wrong,Dark Valefor had 350k of HP,Dark Ifrit had about 600k HP,Dark shiva had
about 450k HP,Ixion you need to defeat it 3 times,it's HP is increaing every time you fight it. :)
I don't think i should tell ya about Dark Bahamut & Anima....cause you may give up playing FFX if i tell
you. :)
So good luck for beating the game^^
Edit: ops...I forgotten to answer ya question...lol....I did kill it :) Good luck^^