In Ascension you only ever use a hero's ability when you play the card. So in your case when you draw the Seer of the Forked Path nothing happens. However on your turn if you play that card you will draw an additional card and get the choice of banishing a card in the center row. The fact that the ability only happens when you chose to play the card is important because there are times when you do not want to activate a hero's ability because it has a drawback that you don't want to encounter.