Tapdown effects are quite common, and indeed have been seen on common equipment like Greatsword of Tyr
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The obvious use case is in Lathril, Blade of Elves, as youll be able to attack, get your elf tokens, and then be able to hold up her tap ability from the beginning of the next combat
In general, their power level is in line with Starter Decks from previous sets
(I always imagine it like being what youd get if chess had certain edge cases where rooks could fly, bishops could teleport, or for a second, pawns were driving tanks.) Since the game is built around several thousand collectible cards, and almost every card has some additional unique ability to it one way or the other, it creates an endlessly manipulable, constantly surprising experience
Who knows