I've just completed the Hagia Sophia challenge level and, as well as Ishak Pasha's armor, got the message Multiplayer bonus unlocked.
This happened once before – after assassinating a particular character, this message appeared and apparently this corresponded to me unlocking that character as a multiplayer character.
I don't have Xbox Live Gold – I can't go into the multiplayer menus at all, so I can't have a look for myself. I've looked at lists of multiplayer characters on the internet and can't find one that either (a) looks like it relates to Ishak Pasha or (b) specifically says it's unlocked like this.
So, what did I just unlock?
Best Answer
The character you assasinated and unlocked was Shahkulu, killing him unlocks the Renegade MP character. He's really easy to spot in the crown and has a distinctive fast-walk animation, making him a really bad choice.
Completing the Hagia Sophia dungeon unlocks an icon shown for your 'templar profile' during MP sessions. It looks like Jack Black with a moustache.
The MP icons are kinda pointless - really it should show your gamer tag icon instead, but I guess it's yet another collectible for you to keep playing for.