If this Nahiri's Wrath is the only card in my hand, can I cast the spell and do 3 damage to one creature, since that is the mana cost of the card. Or do I have to have another card to discard for it to take effect?
If this Nahiri's Wrath is the only card in my hand, can I cast the spell and do 3 damage to one creature, since that is the mana cost of the card. Or do I have to have another card to discard for it to take effect?
You can't discard and cast the same card (barring the Madness ability).
When you cast Nahiri's Wrath, the first thing you do is put it on the stack, then choose targets, then pay costs. Since you put it on the stack first, it isn't in your hand to discard to pay the cost.
You aren't discarding Nahiri's Wrath when you're casting it, so it doesn't count its own converted mana cost for the damage. Also, with no discarded card, X=0 so it can't target anything anyway.
The first thing you do when casting a spell is to put the card on the stack. If Nahiri's Wrath is the only card in your hand, casting it will make you unable to discard any cards, because your hand is empty. So your only legal choice for X is 0, and NW basically becomes a 0 damage spell with no targets. To get it to deal damage, you need at least one other card with CMC above 0 in your hand to discard and designate a target (the targeting can be done with a land card or some other 0 CMC card, though, but NW won't deal damage then).
When you cast Nahiri's Wrath, the first thing you do is put it on the stack, then choose targets, then pay costs. Since you put it on the stack first, it isn't in your hand to discard to pay the cost.
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