Rayic Gethras is a Cowled Wizard that you will have to kill in connection with the Find Proof of Mae'Var's Treachery side quest.

After stealing from the Temple of Lathander or Talos, Mae'Var will direct you to Edwin who in turn will ask you to kill Rayic Gethras for him. Make sure you prepare before fighting him as he can cast a few high level spells including Finger of Death.