Dark Souls

Achievement: Occult Weapon
Description: Acquire best weapon through occult reinforcement

Here is a simple list of mats needed and steps to undertake.

First u will need "Dark Ember" And give it to Andrei (The blacksmith in Undead Parish near the entrance to Darkroot Garden and Sen's Fortress).
The Dark ember you will find in The Painted World of Aremias(Wich is located in Anor Londo. You enter it using the big painting in the chapel of Anor Londo). To interact with the painting and get to the painted world however you will need the Perculiar Doll. Wich can be found in The Northern Asylum. This was also the starting area. You return here by using the bird at Firelink Shrine. I added a video to show you how to get back there. The item lies in the back area. Its not to big so you will find it pretty easy. Just run around the breaking floor to not get engaged into a boss fight.
Be warned tho, Once you enter the painted world you have to complete it to continue.


Once you get the Dark ember to Andrei only the mats will remain. So here are your options.
A. Start from a fresh weapon. Wich requires you to get the following:
+5 Normal
+5 Divine
+5 Occult

B. You wait for a weapon with the occult reinforcement already in place. Wich requires:
+5 Occult

For the mats its pretty simple:
Normal 1-5 : 9 Titanite Shards
Divine 1-5 : 10 Green Titanite Shard (Including the one to tranfer from Normal to Divine)
Occult 1-4 : 7 Chunk of White Titanite (Including the one to Transfer from Divine to Occult)
Occult 4-5 : White Titanite Slab

Doing this will net you the achievement.
If you wonder where to get those mats, well the Titanite Shards and the Green Titanite Shards are bought at the vendor on top on Sen's fortress. Titanite shards for 800 souls Green Titanite Shards for 5000 souls. If the green ones are to pricy for you, you can always farm on the leeches in Blighttown Swamps, or on the Slimes in the depths.
For the White Titanite Slab you will need to venture to the Catacombs into the Tomb of Giants. Right before the boss in that area you will be in a room with 6-7 Magicians and some mini-skeletons(wich are in my opinion the best in-game source of Humanity farming). When you walk the stairs back up towards the end of the room keep a lookout on your left cuz there will be a hole u can fall into wich contains the dead body holding the White Titanite Slab.



A way to make farming a little less painfull is getting yourself the Covetous Gold Serpent Ring(It boosts your Item Discovery to 300 without any humanity's. For each Humanity active on you your discovery will also rise). Wich you can find in Sen's fortress.
When you reach the second boulder contraption run all the way down like you normally would. But instead of heading into the room with the automatic lift wait at the entrance of that room till another bould comes past you and run to the back where the boulders roll to. There you will find the corpse holding the ring.

Hope this helped!

I take no credit for this video

Just to add to Cheps' mention of an already reinforced occult weapon, you can find the Occult Club from killing the mimicry in Anor Londo next to the chests where Havel's set is located.

I can also confirm that NO dark ember is needed if you want to get the achievement from the club. As it already has the occult reinforcement on it, all that is needed is the white titanite chunks and white titanite slab to get it to +5. I never acquired the dark ember in any of my 2.5 playthroughs of the game.

"You can obtain the White Titanite Slab in the Tomb of the Giants just before fighting Gravelord Nito, beyond the Golden Fog door with the baby skeletons, in the area is a corpse laying on the ground containing the White Titanite Slab. Also, it is a very rare drop from Pinwheel Clones in that same area." - LSDintensity.

Here's a video showing you how to get to the correct mimicry if you're a little unsure. Hope this helped! I take no credit for this video.

There is an Occult Club, it is located at Anor Londo in a single Mimic Chest behind a hidden Wall in a Fireplace, where also Havel's Armor can be found too. So the Ember is not needed and it needs less Items to upgrade fully because its already occult, also it is called "Occult Club".