Defeat Dark Sun Gwyndolin
In order to fight this boss you first have to find the Covenant "Blade of the Darkmoon"
Gaist Heidegger has a great guide on how to accomplish that
After you get your covenant achievement walk through the fog wall and you will be treated to a cutscene, then the boss fight begins.
*note that passing through the fog will break your covenant and turn the Anor Londo firekeeper hostile, killing the boss will lock you out of the "darkmoon blade" miracle for that play-through.
Gwyndolin will be at the end of a never-ending corridor and throw 3 different random attacks your way;
he'll raise himself off the ground and fire a few volleys of arrows your way - you can raise your shield and defend or you can hide behind the many pillars along the walls,
his second attack he will raise his staff and fire a group of homing magic arrows - hide behind the pillars against this attack as it will damage you even if you're defending,
his third attack will have him raise his staff and fire a large magic attack - you must dodge this it as it will travel through the pillars.
Once you get close he will teleport down the corridor and continue attacking you so when you're a few pillars away dodge his next attack then sprint towards him and get a few hits in before he teleports. Fire seemed to work very well against him, a high level pyromancy and chaos fireball should make this fight a lot easier.