Thunder Shock is Pichu's Final Smash in Super Smash Unleashed!. Pichu performs a HUGE electrical attack, in which is the most powerful Final Smash in the game, but it comes with a great price: sacrificing one whole stock without resetting damage.
When Pichu uses this attack, it surrounds it inside a huge ball of electricity, and then performs a huge electric storm. This kills all enemies playing in the game, as this pervades the entire stage or server. This gives 1000% damage to all enemies.
The move stuns the opponent for a long time, so they have no way to escape the move. As they are stunned, their health will drain from too much electricity.
However, using the most powerful Final Smash comes with a big price: Pichu must sacrifice one whole stock, and its starting damage of its next stock will stay the same number as before it used Thunder Shock. This means, as an example, if Pichu is at 107% damage on two stocks, he will lose a stock and will start with 107% damage when he is done performing Thunder Shock.