There are MANY weapons in Dark Souls. All of them are useful for something in their own merit, but some of them are considered rare, and are needed for this achievement. To obtain all of the rare weapons, you must do a minimum 3 playthroughs for boss souls as well as cut the tails off applicable bosses and scour the world for all of the weapons. When you have the necessary ingredients to make these weapons, take them to the giant blacksmith in Anor Londo. Some general tips:

- NEVER consume a boss soul unless you are sure you don't need it for anything.
- You do NOT need to upgrade any of these weapons, but if you like the weapon, I encourage you to max out it's upgrade path.
- The Giant blacksmith will stay there in Anor Londo even if it is dark.

Boss Soul Crafted Weapons

Moonlight Butterfly Horn - Soul of a Moonlight Butterfly, any +10 spear or thrusting sword
Crystal Ring Shield - Soul of a Moonlight Butterfly, any +10 shield
Quelaag's Furysword - Soul of Quelaag, any +10 curved sword
Chaos Blade - Soul of Quelaag, any +10 katana (recommend using Washing Pole as it's the worst katana, in my opinion)
Greatsword of Artorias (CURSED) - Soul of Sif, any +10 dagger, straightsword, or greatsword except Broken Straight Sword and Straight Sword Hilt
Greatsword of Artorias - Soul of Sif, +10 Straightsword Hilt or Broken Straight Sword
Greatshield of Artorias - Soul of Sif, any +10 shield
Golem Axe - Core of an Iron Golem, any +10 axe
Dragon Bone Fist - Core of an Iron Golem, any +10 fist weapon
Lifehunt Scythe - Soul of Priscilla, any +10 Halberd or Whip
Dragonslayer Spear - Soul of Ornstein (kill Smough first, then Ornstein), any +10 spear or thrusting sword
Smough's Hammer - Soul of Smough (kill Ornstein first, then Smough), any +10 hammer
Darkmoon Bow - Soul of Gwyndolin, any +10 bow
Tin Darkmoon Catalyst - Soul of Gwyndolin, any catalyst EXCEPT Demon Catlayst, Izalith Catalyst, Manus Catalyst, and Tin Darkmoon Catalyst
Great Lord Greatsword - Soul of Gwyn, any +10 dagger, greatsword, straightsword

Tail Cut Weapons

Drakesword - Cut the tail of the Hellkite Dragon (red dragon guarding the bridge in Undead Burg). I recommend getting a bow and ~35 arrows and going beneath the bridge and sniping the tail off with your bow as it's a trick tail cut with a melee weapon.
Dragon King Greataxe - Cut the tail off of the Gaping Dragon (boss in the Depths, requires key to Depths dropped by Capra Demon boss)
Priscilla's Dagger - Tail cut from Priscilla. This is one of the trickiest tail cuts. I recommend getting your hardest hitting weapon and then doing a two-handed strong attack while she isn't hostile.
Moonlight Greatsword - Tail cut from Seath the Scaleless. If you're having trouble with this, stand by the crystal that regenerates his health and wait for him to summon the crystals. When he does this, he'll destroy the crystal and be stunned for about ~10 seconds, then run and attack the tip of his middle tail. It may take a few tries, but it's the easiest way to get your hard-earned sword.
Dragon Greatsword - Cut the tail off of the Everlasting Dragon at the end of Ash Lake. I recommend doing this after you get the Lordvessel so you don't have to run up the Great Hollow again. Either way, the dragon can't get hostile and won't fight no matter how much you hit, so just go hit the tail a couple of times and the sword is yours!
Gargoyle Tail Axe - Cut the tail off of the Belfry Gargoyle. This is the easiest tail cut since the tail is long and weak. If you miss this in the Undead Parish, you can cut the tail off of the FIRST gargoyle in Anor Londo.

Covenant Weapons (Weapons gained from joining covenants)

Gravelord Sword - Join the Gravelord Servant covenant. See "Covenant: Gravelord Servant" on how to do this.
Dark Hand - Join the Darkwraith covenant OR uncommon drop from the Darkwraith enemies in New Londo Ruins. See "Covenant: Darkwraith" for how to do this.

Enemy Dropped Weapons

Black Knight Sword - See 'Black Knight Shield' bullet point for where to farm these weapons.
Black Knight Greatsword - See 'Black Knight Shield' bullet point for where to farm these weapons.
Black Knight Greataxe - See 'Black Knight Shield' bullet point for where to farm these weapons.
Black Knight Halberd - See 'Black Knight Shield' bullet point for where to farm these weapons.
Black Knight Shield - At the end of the game, you are faced with an area known as Kiln of the First Flame. The black knights here all wield these weapons and are easy to kill, and most importantly, they respawn. Have 10 loose humanity and where the Covetous Gold Serpent Ring to maximize item discovery and minimize the farming time.
Channeler's Trident - Dropped by Channelers. The first one you find is in the church of the Undead Parish. It's the enemy that dances around and makes a taunting sound at you. The best way to farm this is to kill the two channelers that respawn in Duke's Archives. Again, have 10 loose humanity and where the Covetous Gold Serpent Ring to minimize the farming time.
Crescent Axe - Dropped by Patches. Forgive him after he kicks you in the ditch (pick up the Skull Lantern in front of you since it makes the Tomb of the Giants less of a pain), exhaust his dialogue, and kill Nito to make him move to Firelink. He'll sell this weapon after he's in Firelink. (You can kill him and he will drop the axe, but he sells some spells to you after he moves, so I wouldn't recommend killing him.)
Crest Shield - Dropped by Oscar of Astora (the knight who gives you the Estus Flask) on your return to the Undead Asylum. (I'd recommend killing the Stray Demon for the titanite slab and picking up the peculiar doll from the cell you started the game in for the Painted World of Ariamis)
Demon's Catalyst - Dropped by the Demon Firesage boss.
Giant's Halberd - Uncommon drop from the Royal Sentinels in Anor Londo. I'd recommend farming the ones at the beginning of Anor Londo since the ones outside Ornstein and Smough's boss room are considerably harder.
Izalith Catalyst - Dropped by the Daughter of Chaos (pyromancy wielding enemy wearing the Gold-Hemmed Set) right in front of the Bed of Chaos boss fog.
Silver Knight Straight Sword - Dropped by Sword- Wielding Silver Knights in Anor Londo.
Silver Knight Spear - Dropped by Spear- Wielding Silver Knights in Anor Londo.
Silver Knight Shield - Dropped by Silver Knights in Anor Londo. It's a rare drop, so boosting your item discovery with the Covetous Gold Serpent Ring and 10 loose humanity is highly recommended.
Stone Greatsword - Dropped by Stone Knights in Darkroot Garden or sold by Shiva of the East for 15000 souls. (Shiva of the East will only show up if you're in the Forest Hunter Covenant and have exhausted his dialogue in Darkroot Garden, then he'll show up in Blighttown and sell this weapon amongst others)
Stone Greatshield - Dropped by Stone Knights in Darkroot Garden.
Grant - See 'Sanctus' below.
Sanctus - This shield and the weapon Grant are both guaranteed drops by Paladin Leeroy. Paladin Leeroy will invade you on your way to Nito (must be human and Nito must be alive for him to invade) whether or not you summoned him for Pinwheel or not.

Corpse/Chest Looted Weapons

Dragon Crest Shield - Picked up from a corpse right in front of the Undead Dragon in Valley of Drakes.
Havel's Greatshield - See 'Dragon Tooth' below.
Dragon Tooth - This weapon and Havel's Greatshield are both located in Anor Londo. From the bonfire in the room with Solaire, go past the sword wielding Silver Knight and go into the room on the left, then hit the fireplace to open up an illusory wall. These can be found on one of the mimics down there.
Dragonslayer Greatbow - This can be found in Anor Londo. In the room with Ornstein and Smough's fog gate, go up the stairs and drop down through the window. The corpse contains this bow.
Velka's Rapier - Found in the Painted World. First, get the Annex Key (found at the base of the well and then straight ahead to the end, then make a left and loot the corpse. BEWARE OF PINWHEEL SKELETONS!!!), then open up the door to the Annex. The Annex is the building to the right of the statue in the center). Go up the stairs, then hit the wooden planks, then go straight to the corpse and loot it for the weapon.
Astora's Straight Sword - Found on the corpse right next to the Dragon Crest Shield.
Bloodshield - Found on a corpse in front of the Undead Dragon in the Painted World.
Black Iron Greatshield - Before entering the painting in Anor Londo for the Painted World, look left. Then loot the corpse you see for the shield along with the black iron set.
Effigy Shield - This is found in the Tomb of the Giants. On the plateau with the two skeleton beast and the skeleton archer there is a slope leading down. There is a Black Knight guarding the way to the corpse so kill him or circle around and go straight to the corpse for the shield.