Madame Web, Clairvoyant is a uncommon Legendary Creature — Mutant Advisor card from Through the Omenpaths (OM1), collector number 36.
You may look at the top card of your library any time. You may cast Spider spells and noncreature spells from the top of your library. Whenever you attack, you may mill a card. (You may put the top card of your library into your graveyard.)