The following is a list of Boss Souls Weapons.
To do this you must first give the Large Ember (found in The Depths) to Andre. To construct a Boss Soul Weapon you must do the following:
You can also consume the Boss Souls if you wish, but they are best reserved for the construction of these unique weapons - especially if you are going for the associated achievement/trophy. There are multiple weapons which can be constructed from the souls (usually two). Boss Soul weapons are weapons which can be crafted from the souls that are dropped by some bosses throughout the game.