Great Magic Weapon is a Sorcery in Dark Souls 2. To cast a Sorcery, you must use a Staff or Special Weapons that can cast Sorcery.
Great Magic Weapon
"A sorcery superior to Magic Weapon. Reinforces weapon in other hand with intense magic, allowing for infliction of heavy magic damage.
The old purists of the Melfian Magic Academy view swordplay as a barbaric form of engagement."
- Company of Champions Rank 1 reward.
- Purchased from Royal Sorcerer Navlaan by 6000 souls.
- Found in a chest in Black Gulch.
- Buffs weapon with magic damage. Lasts roughly 60 seconds at 18 Intelligence. Preceded by Magic Weapon and succeeded by Crystal Magic Weapon in terms of damage.
The duration is directly related to intelligence. If you use Simpleton's Spice to reduce the cost to 10, the spell will only last roughly 20 seconds.