To craft special weapons from boss souls in Dark Souls 2, you need to rescue two hidden NPCs. The first NPC is Straid of Olaphis, who needs to be unpetrified by a Fragrant Branch of Yore. You receive a Fragrant Branch of Yore after killing The Lost Sinner boss.
The second NPC is Weaponsmith Ornifex, who needs to be rescued from a jail cell in the Shaded Ruins. You get the jail cell key by unpetrifying a specific enemy nearby. After freeing Ornifex, you can find her in her house down in Lower Brightstone Cove.