Nahiri's Wrath is a mythic rare in EMN.
There are 14 mythic rares in EMN, discounting Brisela and wolfmode Ulrich, both of which are the back faces of other mythics in the set.
The chance of a random EMN mythic being Nahiri's Wrath is 1/14.
The chance of a random booster pack containing a mythic rare (discounting DFC shenanigans) is 1/8.
Therefore, again discounting DFCs messing with probabilities, the chance of a random booster pack containing a nonfoil Nahiri's Wrath is 1/14 * 1/8 = 1/112 ~= 0.89%.
The actual probability of getting Nahiri's Wrath is likely a little higher than that, as each pack contains a foil card that could be of any rarity. I don't know how that probabilities of different rarities are distributed in that slot, though. I also don't know how the DFCs replacing the card in the rare slot factor in... In SOI, I think, one in eight packs had a rare or mythic DFC replacing the card in the rare slot, and in EMN, it's probably the same. But I don't know precisely how they tweaked the probabilities. If the probability of getting Ulrich, say, is the same as that of getting Nahiri's Wrath, and the same is true for rares, then the above math should work. But it might not be, so I don't know.
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Half of a wall o' text I posted in Discord.