If the targeted opponent is an illegal target by the time Cerebral Eruption resolves, the spell doesn’t resolve. No cards will be revealed and no damage will be dealt.

Once the card is revealed, it’s too late for players to respond. The targeted opponent can’t, for example, see how much damage will be dealt, then cast a damage prevention spell or activate a regeneration ability. All such actions must be done before Cerebral Eruption starts to resolve.

The mana value of the revealed card is determined solely by the mana symbols printed in its upper right corner. The mana value is the total amount of mana in that cost, regardless of color. For example, a card with mana cost {3}{U}{U} has mana value 5.

If the mana cost of the revealed card includes {X}, X is considered to be 0.

If the revealed card has no mana symbols in its upper right corner (because it’s a land card, for example), its mana value is 0.

If a land card is revealed this way, Cerebral Eruption is returned to its owner’s hand from the stack. It finishes resolving, but it isn’t put into the graveyard.