Spellweaver

History
This ancient elven-forged blade vibrates slightly to the touch in most people's hands, but it stills in the grip of a mage. Crafted with lost techniques for alloying lyrium and metals, Spellweaver enhances a mage's powers, acting as a focus in the same way as a staff.

Location
A drop from the Arcane Warrior Cultist Overseer in the north-eastern side of the Mountainside Caverns in the Ruined Temple, during the quest line for the Urn of Sacred Ashes.

Spellcasting script
Unlike other Longswords, the Spellweaver allows the Arcane Warrior to cast spells that normally require the mage's weapons to remain sheathed, similar to Magic Staves. (See Arcane Warrior Spellcasting for more info)

Actually the script (if it exists at all) doesn't work. An arcane warrior casting "Cone of Cold" for example needs to sheathe its weapon first, which is a waste of time.

It is possible that the "enhances magic abilities" mentioned in the item description actually refers to the magic bonus. The sword may not have been meant to allow spells to be cast, sword in hand, other than those that can be cast with any other sword in hand.

Xbox version

 * In the Xbox version, Spellweaver possesses +2 Combat Mana Regeneration.