Spellweaver

History
Acquired by killing the Cultist Overseer in the eastern cave of the caverns during the search for the Urn of Sacred Ashes.

Can also be found in the Elven Burial Chamber in the Brecilian Ruins, from the Shade. It seems you need to have completed Castle Redcliffe first to have this happen. (needs confirmation)

Trivia

 * The description for this weapon states that the arcane warrior can cast spells while wielding this sword. This, however, is not true. The Spellweaver does not allow the mage to cast spells while using this sword; The mage will simply sheathe the sword, cast the spell and unsheathe the sword again.