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Note that this doesnt restrict you to discarding just Metal Energy cards its any type of Basic Energy

While this card is in Attack Position, your opponent takes any effect damage you would have taken from their card effects, instead.

Cascoon from EX Legend Maker (2006)

If a Gladiator Beast attacks Don Zaloog , they have to discard a card. Then, if the opponent Special Summons Gladiator Beast Murmillo to destroy Zaloog, Nolan will just attack Murmillo next turn with a Rose Token and destroy it

Before Pendulum, his strategy of effectively blocking his opponent's Spell/Trap when comboing with Dark Simorghwas too famous