アーティファクト・クリーチャー — 猫
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, あなたの手札を捨てる, ダイヤモンドのライオンを生け贄に捧げる:好きな色1色のマナ3点を加える。起動はインスタントとしてのみ行う。
You can't cast an instant (or activate an ability as an instant) while casting a spell. Therefore, much like Lion's Eye Diamond, you can't activate the ability intending to use the mana to cast a spell from your hand.
Even though you can only activate this as an instant, it is still a mana ability. It doesn't use the stack and cannot be responded to.