Killing bosses yields boss souls that hint that they can be used to create new weapons, however there is no available option to do so at the blacksmith. How do you create boss weapons in Dark Souls II?
In DSII, you no longer ascend a weapon to a Boss weapon using a boss soul. Instead you 'trade' the boss soul and some additional souls to 2 specific NPC's to get Boss weapons.
The first is Straid of Olaphis who can be found petrified in a jail cell guarding a bonfire at the top of Lost Bastille. Using a Fragrant Branch of Yore will allow the player to trade boss souls with him. To be able to trade with him, you need at least 3 Intelligence and 3 Faith. This is a one-time requirement. If you later redistribute your stats with a Soul Vessel, you can still trade with him so long as you have talked to him with the required stats at least once.
The second is Weaponsmith Ornifex but you must unlock her before you can trade with her. To unlock her visit Shaded Ruins, to the right of the bonfire is a petrified enemy, you can jump from the ramp onto the raised walkway and use another Fragrant Branch of Yore on him and he will drop the Fang Key. After getting the Fang Key, drop down in the pit next to the giant Green Basilisk and use the key to open the door. This is the first time you meet Ornifex and she will tell you she is moving to her home.
She will then move to Brightstone Cove Tseldora, in the room next to the Brightstone Cove Bonfire. Talking to her here finally allows the player to trade the rest of the boss souls to her.
Keep note that depending on how many bosses you have fought before unlocking Ornifex, will be the number of boss weapons she has. She will not show you boss weapons for bosses you have not beaten yet so always come back to her as she will craft the weapons for all bosses in the second half of the game. Also, once you free her, Ornifex will perform one, and only one, Boss Soul trade for free.