This achievement requires upgrading a standard weapon to Occult +5. The general procedure would be to have Andre in the Undead Parish upgrade a weapon to +5, then to Divine +5, then to Occult +5. This would require the Divine Ember from the tower after the Moonlight Butterfly, the Dark Ember from the Painted World of Ariamis, and a total of 9 Titanite Shards, 10 Green Titanite Shards, 8 White Titanite Chunks, and 1 White Titanite Slab.
The easiest way to get this achievement, though, is with the Occult Club found in Anor Londo, which any blacksmith can upgrade to +5 with 7 White Titanite Chunks and 1 White Titanite Slab.
Titanite Shards and Green Titanite Shards are found in limited numbers as you progress and can be bought from some NPCs. White Titanite Chunks can be found in somewhat limited static locations throughout the mid-to-late stages of the game (as guaranteed enemy drops or on corpses). The fourth Black Knight in the Kiln of the First Flame (the final area of the game) respawns and always drop a White Titanite Chunk. You can find a White Titanite Slab in the area immediately before Gravelord Nito in the Tomb of the Giants. A second one can be found just before Manus in the Chasm of the Abyss, but that area of the game is optional.
Divine and Occult reinforcements use the same upgrade materials. The game only saves when you back out of an upgrade menu, so you can get these two weapon upgrade achievements with a single White Titanite Slab and 7 White Titanite Chunks if you'd prefer to not have to gather 2 Slabs and 14 Chunks. You'll end up losing the resulting fully upgraded weapon by dashboarding right after the achievement pops (before backing out of the upgrade menu) but you will get your upgrade materials back.
You'll definitely want the Occult Club from Anor Londo, which already has the Occult infusion applied. This way you won't have to bother with upgrades to get to that point which would normally require 9 Titanite Shards, 10 Green Titanite Shards, and 1 White Titanite Chunk. For the Divine Weapon achievement, you'll need the Large Divine Ember found in the Tomb of the Giants, but you won't actually need the Divine Ember from after the Moonlight Butterfly (at least I don't thin you need this, but you should grab it anyway just to be safe) or the Dark Ember from the Painted World since the Occult Club is already upgraded beyond what Andre would need those two embers to do.
Once you have everything you need, head to Andre in the Undead Parish and give him the Large Divine Ember. Start by reinforcing the Occult Club to +5. Wait a few seconds after applying the final upgrade (which requires 1 White Titanite Slab) and this achievement will pop. Again, do not back out of this menu screen confirming that the weapon was upgraded to +5, but instead go to the dashboard and force quit the game, then restart the game and continue. You'll be back at Andre as if you had never done the Occult upgrades.
Now have Andre "Modify Equipment." The far right option should allow you to revert the Occult Club to a +5 Divine Club. Then choose the option to upgrade ("ascend") the +5 Divine Club to +6, which requires the Large Divine Ember. Back up to Andre's main menu and choose to Reinforce Weapon, then upgrade the +6 Divine Club up to +10, at which point the Divine Weapon achievement will pop. You can dashboard here if you'd prefer to save your Chunks and Slab for something else.
As always, refer to
http://darksouls.wikidot.com for specific information relating to items or locations.