Obtain an Elixir of Shadows from Severin and use it to find and slay Talonpriest Ishaal, Talonpriest Skizzik and Talonpriest Zellek in Skettis. Return to Commander Adaris after completing this task.
Thank you for paying heed to my fevered rants. All along we had been trying to identify and destroy the Skettis chain of command; we had learned of three clerics known as the talonpriests, but we'd never seen them with our own two eyes. Now we know why.
Talk to Severin to obtain more of the elixir he discovered. Use the elixir to find and kill the three talonpriests: Ishaal, Skizzik and Zellek. They're rumored to dwell in Veil Ala'rak, near the eastern shore of Blackwind Lake.