How To Become Gwyndolin

You will have to playthrough the game twice to become true Gwyndolin.

First You want to start off by making a character that has some resemblance to Gwyndolin.
By the end of this you will want to have 50 in Vitality, Dex, and Faith. Play through the game as usual. After defeating Ornstein and Smough attack Gwynevere to destroy the Illusion.
After, head back to the turning tower. Crank it all the way down to find where Gwyndolin is hiding. After defeating Gwyndolin, you can head back to the bridge next to the Firelink Shine to buy Gwyndolins armor set from Domhnall of Zena. Once you have his set, get any bow upgraded to plus 10. (Note you will need the Large Ember to get weapons to plus 10. It can be found in a chest next to the butcher in the first room of the depths.) Now that you have a plus 10 bow, go to the Giant Blacksmith to upgrade the bow using the soul of Gwyndolin to transform it into the Darkmoon bow. Buy all the Moonlight Arrows. Upgrade the Darkmoon bow to plus 5 using Demon Titanite.

Now that you have the Armor Set, bow, and arrows, all you need is the Darkmoon Talisman. However, in order to use Darkmoon Blade, you need Gwyndolin to be alive and in great standing. TIME FOR NEW GAME PLUS!

Start a NewGame+ after parrying Gwyn all the way back to Deamon's Souls. Play through the game as you did last time, however, after defeating ornstein and smough do NOT attack Gwynevere. She is Friend now. Travel to The Catacombs and go through as normal. you will see a breakable wall. Roll through it and continue forward.

Soon after, you will find a giant skeleton jumping down in front of a coffin with a item inside. There you will find the Darkmoon Seance Ring. Wear it and warp back to Anor Londo. From there, go back down the turning tower into Gwyndolins room and equip the ring infront of the statue. This will cause it to fade away. Go up to the boss fog but don't enter or all will be for not. Instead, kneel at the rug and join the Blade of the Dark Moon Covenant.

FARM FOR 80 SOUVENIRS OF REPRISAL!

Just invade players who have committed sins and kill them 80 times. If that's no fun or too hard, just go to the Painted World Of Ariamis and farm the Crow Deamon for Souvenirs Of Reprisal. (only 6% drop chance tho ☹) DO NOT ABANDON COVENANT OR JOIN ANOTHER BECAUSE IT WILL REMOVE HALF YOUR OFFERINGS!. After giving over the 80 Souvenirs Of Reprisal, you will be granted the Darkmoon Talisman and the Darkmoon Blade, as well as get two power boosts for the spell. If Gwyndolin or Gwynevere are harmed, or you enter his boss arena you will lose the ability to cast Darkmoon Blade.

Beat up this guy in Undead Parish until he drops the Balder Side Sword (it has a 1% chance to drop, good luck.) After obtaining the Balder Side Sword, upgrade it to plus 15. (You need the Very Large Ember to get past plus 10. This is found in a chest in New Londo Ruins in the main building) Casting Darkmoon blade on this sword makes for a nasty dex weapon.

Congratulations! You now have Gwyndolins build. Throw on that FAP Ring and a ring of your choice and show the hollow the true power of the Dark Moon!