The Blasphmous Blade is obtained by trading the Remembrance of the Blasphemous with Enia at the Roundtable Hold. This Remembrance is obtained after defeating Rydark, Lord of Blasphemy, the Demi-god boss of Mt. Gelmir. Ths boss fight is relatively easy as you find the Serpent-Hunter just before encountering him, a weapon that is designed specifically for putting him down – the boss fight itself seems to be intended for flashy purposes, in all honesty.