DARK SOULS II
38 Achievements
1,000
Supreme Weapon
Reinforce a weapon to its limit
50
How to unlock the Supreme Weapon achievement in DARK SOULS II - Definitive Guide
When it comes to collecting the materials, large and small titanite shards are easy enough to come across. Titanite chunks can be gained by joining the Bell Keeper's covenant in Belfry Luna. You'll earn a titanite chunk for each "invader" of the tower that you successfully kill. As for the slab, it can be find behind a secret wall next to a Pharros' Lockstone spot in the Forest of Giants. This is right below the area with hollow soldiers manning a few ballistae, close to where Pate can be found, and a little after the second bonfire.
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+1 = 1 T(itanite). shard
+2 = 2 T shards
+3 = 3 T shards
+4 = 1 Large T shard
+5 = 2 Large T shard
+6 = 3 Large T shard
+7 = 1 T chunk
+8 = 2 T chunks
+9 = 3 T chunks
+10 = T slab
This can be done on any base weapon that you see requires Titanite to upgrade and not Twinkling Titanite, so things like a Longsword or Shortsword are good options. There is a Titanite Slab pickup in the Forest of the Giants level, make sure to grab it so that you don't have to farm one from enemies later.
You will need the following to max out an item: 6 Titanite Shards, 6 Large Titanite Shards, 6 Titanite Chunks, 1 Titanite Slab. Most of these are found through natural progression. A slab can be found in the Lockstone room below the Ballista room of Forest of Fallen Giants. Of course you will also need the required amount of souls to upgrade the weapon. The easiest blacksmith to find is Lenigrast in Majula. Speak to him and buy his shop key from the merchant by the Cardinal Tower bonfire. Then head back and open the store. Once you upgrade a weapon to +10 with the above items this will unlock.
To get a weapon to +10, you will need 6 Titanite Shards, 6 Large Titanite Shards, 6 Titanite Chunks, and 1 Titanite Slab. Shards, Large Shards, and Chunks can be purchased in large quantities by various merchants throughout the game, but not until many hours in. You should have enough Regular / Large Shards by progressing through the game, but the fastest way to gain Chunks is through the Bell Keepers Covenant, as they award 1 Chunk for every successful "invasion".
To get this trophy you need to reinforce one of your regular weapons or shields to +10 or reinforce special boss soul weapons/shields to +5 at any blacksmith in the game. Reinforcing a regular weapon to +10 is much easier since it only requires the 4 forms of titanite versus the petrified dragon bone stones required to upgrade boss soul weapons. What I recommend is find a weapon you like and start upgrading it, below is what you'll need to upgrade a weapon to the max:
6 Titanite Shards
6 Large Titanite Shards
6 Titanite Chunks
1 Titanite Slab.
You can purchase all of those items after you've defeated the final boss from Stone Trader Chloanne in post game when she is in Majula. So do not stress if you don't find the titanite throughout the game.