First of all, you need to forge a weapon to +5 status (using Titanite shards, which can be purchased from a Blacksmith), and then transform it into a Raw Weapon.
Once it becomes a raw weapon, you need to forge it to +5 again and the Achievement is yours!
You will need, in total:
Titanite Shards x9
Large Titanite Shards x10
Only Andrei in the Undead Parish can upgrade to Raw Weapons.
He can be found to the right of the church in the Undead Parish, into the building, then down the stairs from the bonfire.
You will also need to provide him with a Large Ember. This can be found in the Kitchen in The Depths. (thanks Torash!)
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