When Cerebral Eruption resolves the targeted opponent will reveal the top card of his or her library. Cerebral Eruption will then deal damage equal to the converted mana cost of the revealed card to that player and all creatures he or she controls. If a land card is revealed this way, then no damage is dealt and the Cerebral Eruption is returned to its owner’s hand.

It is possible for Cerebral Eruption to deal zero damage and not be returned to a hand. If a card with a converted mana cost of zero is revealed; like Accorder’s Shield, Memnite, or Chimeric Mass then Cerebral Eruption will not deal any damage and also not be returned to its owner’s hand. If Dryad Arbor were to be revealed, the Cerebral Eruption would be returned to its owner’s hand as Cerebral Eruption only cares if a land is revealed, the fact that it is another card type does not matter.