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.