Use a star stone.
Is this during the trial to join the immaculates, or did you find some tenebrium without having the Tenebrium skill? If the former, keep going through the trial; if the latter (which it sounds like), see if you have picked up any unused star stones (if not, keep healing yourself until you find one).
Brandon in Silverglen can teach you the Tenebrium skill if you help him with something, or you can find a book to teach it in Sacred Stone.
