Okay now once you make your way off to the right of the church that has the Bell Gargoyle perched on top you will find a long bridge surrounded by thick woods just make your way down the tower at the end, light the bonfire for a checkpoint and continue down and meet the blacksmith who will repair and enhance your weapons for the fair price of a few souls and titanite shards.
Now go down the stairs and fight (or run past) the demon there and continue on through the forest, be wary of the walking bush monsters (lol.) And the path to the moonlight butterfly is a pretty straight forward one just watch out for the stone knights. Now IF you happen to be in human form you can summon a NPC to help you fight the moonlight butterfly though I don't think you will necessarily need it as it is not a hard boss. Climb the winding staircase with or without your phantom and step through the fog/white light and meet The Moonlight Butterfly. Now even though it is a Butterfly it still does have a lot of decent attacks that do a hefty amount of magic damage if you are slow on your feet to dodge them.
One of his main attacks will be 6 soul spears that shoot out in all directions this is a little hard to dodge since it takes up most of the small bridge you will be fighting on but it doesn't hurt as much as the other attacks. Speaking of he also shoots out 5 slow moving soul orbs but they do track you and if you fail at dodging them they could all potentially could hit you at once taking out major chunks of your health or if your a weak character possibly kill you. Another slow but strong move is an energy bomb, you will know he is doing this when he (it) flies above the bridge and a large blue ball starts to grow under him and he drops it down onto the bridge taking up a large area and doing major damage. One of his other moves is a large Ray beam he starts to fly side to side far from the bridge and a small bright light glows from his...chest and shoots out towards the bridge moving side to side on it it is easily dodged by rolling through it. And finally his last attack is a face bomb he will land on the bridge (also an opening for retaliation) and a bright light glows from his face and explodes does half health if your at ground zero.
All in all he (it) isn't the hardest optional boss out there but if your a hunter or have high (above 15) dex with a good bow you can do some major damage if your a pyromancer your fireballs can reach him from the bridge or if your none of the above always remember the NPC she is a mage and shoots soul arrows at the thing. But once its dead you get a hefty amount of souls passage to get the large embers for the blacksmith and the Soul of the Moonlight Butterfly.
HINT: The soul of the moonlight butterfly can be used to make one of two things but you must visit the giant blacksmith of Anor Londo to get them.
1: Horn of the Moonlight Butterfly. It is mainly a magic damage dealing weapon but to get it you need a spear weapon upgraded to +10 the perfect weapons are; Mail Breaker, Estoc, Rapier, Ricard's Rapier, Spear, Winged Spear, and the Pike.
2. Crystal Ring Shield. It is mainly a magic and critcal defense weapon to get it you need any shield upgraded to +10 and to upgrade it to its full potential you need 25,000 souls and 10 demon titanites.