Text (CH+errata): ~this~ deals damage equal to the number of cards in target player's hand to that player. [Oracle 2009/07/08]
The number of cards in hand is determined on resolution. [Duelist Magazine #2, Page 9] So, your opponent may cast spells in their hand before this spell is resolved in order to take less damage.
Extended tournaments (see Rule 913) have banned this card since 1999/10/01.
