In addition to its rarity, Arthur is quite strong, with an ability that is likely to be leveraged by many serious Lorcana players
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Then this drops on the second turn and then will last as long until killed by a foe, and if you went first, then Arcane Signets and Mind Stones are getting tapped or you get a free 1/1 and then on the third turn you get Manalith, and then don't forget this is dorks too, so everything from Reclamation Sage to Sakura-Tribe Elder to Mulldrifter will ETB tapped for blocking purposes unless you get a free Vampire
Opposing +1/+1 Counters decks suddenly grow at half speed, making their payoff Creatures feel painfully underwhelming next to yours
But timed right, this ability can end games