You need to get a +5 Chaos weapon. Chaos weapons scale with the amount of soft humanity you have (geared towards PvP).
Chaos Fire Ember: Found in the Demon Ruins. This will be on a corpse in the middle of the lava past the Taurus Demon mini bosses. You can only access this part of the map after you have killed Ceaseless Discharge. I recommend having the Orange Charred Ring (defeat the Centipede Demon boss battle) or by putting on Flash Sweat and having high health. I recommend the former. After you have obtained the ember, bring it to Vamos the skeleton blacksmith in the Catacombs.
You must first obtain a +5 normal weapon, then ascend it to a +5 fire weapon, then ascend it to the chaos path. When all is said and done, you need the materials below.
Titanite Shards x9 - Purchased from Andre of Astora in the Undead Parish.
Green Titanite Shards x10 - Purchased from the giant blacksmith in Anor Londo.
Red Titanite Chunks x8 - Farmed from Chaos Eaters in Lost Izalith or a guaranteed drop from the greatsword wielding Black Knight in the Kiln of the First Flame.
Red Titanite Slab - Obtained from a chest in Lost Izalith (VIDEO) or trade an upgraded Ascended Pyromancy Flame with Snuggly the Crow.
After you have obtained a +5 chaos weapon, the achievement will unlock.