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But Arius costs 5 mana.

Look at the Archmage and Rennyn. Both have the "same" attributes, both cards are Legendary, the bonus of the Archmage is the Dragon Splinter.

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If you now compare Julian with the other 2 Mana Splinters, you will notice that they have the same attributes, but the 2 Mana Splinters are a rarity level among Prince Julian. Either the epic 2 Mana Splinters are too strong, or Prince Julian is too weak.

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But you cannot use dragons with any other 2-Mana-Summoners - that's in every case a benefit of Prince Julian. So Prince Julian is in every case the best card for "Summoner give no buff"

I don't think rarity should (or does) determine how "good" a card is. Perhaps rarity is for more exciting cards.

Rarity is irrelevant for Julian. It is only available at max strength. Other than changing the price/bcx number, it wouldn't matter even if Julian were a common.

And, anyways, given how many are going to be produced -- Legendary is absolutely the correct "rarity".