Daedric Influence

Acquire a Daedric Artifact - one of the unique items from a Daedric Prince's quest. An easy early one is the Mace of Molag Bal (do 'The House of Horrors' in Markarth), or Azura's Star (the shrine of Azura east of Winterhold). Any Daedric quest reward counts; feeds Oblivion Walker (Oblivion Walker).