Archcanon Tarvus sends you to talk to Barilzar, a mage who was seen in the ritual you preformed for Lord Vivec. Barilzar works out of an ancient Velothi tower. His experiments radiate energy similar to Vivec's affliction, but they're not a perfect match.
The so-called "Velothi Towers" were built by Chimer in the early days when Morrowind was known as Resdayn. Many of these remote outposts have been occupied by hermits, wizards, and others who value their privacy, including the mage Barilzar.
- For the answer to the Barilzar's Tower Puzzle, see the following entry: Barilzar's Tower Solution