Crystal 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.
Crystal Magic Weapon
"Imbues weapon in other hand with crystal magic. More powerful than standard magic reinforcement.
The utilization of crystals to enhance soul power is an ancient form of magical reinforcement."
- Chest in Dragon Shrine. Above two Drake Keepers; one with a mace and one with a sword and shield. To get it, run over the small bridge on the right and continue through that path. Eventually you will end up above the ledge with the chest. Just drop down and the chest should be on the right.
Rare drop from Ducal Spiders
- Adds 50 magic damage to the weapon, overall magic damage done is increased by 30% for 90 seconds.
Requires a Staff in left hand to apply to right hand weapon, or vice versa.
You can use Blue Flame to cast this on another weapon.