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?