It isn't the card itself that is great but what the card proves about metagame and prereleases. It is a testament to how choosing a card for the prerelease promo and then advertising that card will be drive prerelease numbers, while keeping it secret keeps the numbers relatively flat or within a low forecast. Of course, this only works if the card selected is sought after, as Alteria was. Choosing a card deemed lame and advertising it will very likely have the opposite effect. So, keep choosing cards that players are looking to obtain for the promo and have some confidence in the product
Promos get selected weeks/months in advance so it is unknown if it was serendipitous that a card was chosen to be the promo and then everyone wanted it, or if TPCi noticed everyone would be on the lookout for this card and then it was made a promo. Either way, people attended DRX prereleases just for this card.
Metagame-wise, it was a flop. Its attack is not worth mentioning. When it isn't taking up room on the bench, because it has 70HP it is getting OHKOed by Pokémon that didn't know they could OHKO anything not named "tynamo"
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It was paired with Garchomp. Together, they require a lot of pokemon on the bench and many of them have low HP. As a Dragon-type, it is searchable by Gabile so you can get the Flight Song Ability into play fast. That is probably not doing it any favors though because you won't be attacking with it so the type never comes into play unless it is being attacked by another Dragon-type. Its previous :colorless: versions (with onyl 1 exception) had :fighting: resistance, which would have helped it fair a little better in the metagame at the time.