MArcomage

great mechanic

Indeed. If you don't do something about the Wraith, it will come back to haunt you stronger than ever :D

good only against zero cost decks in this form
maybe " If matching card cost is <20 next card will be Wraith" will be better idea

It's not too bad as an Undead even without the effect. 19 attack for 20 gems is a poor value, but triggering the Undead keyword to get 7 gems refunded and then playing Eternal guardian to benefit from N=20 would make it worthwhile, and there's always the chance that the opponent draws a 0-cost that happens to be in their deck, not just a 0-cost deck. Or you could include it in a deck where you're replacing the opponent's cards with 0-cost.

The real drawback, as I see it, is that it's going to be difficult to play a card that costs 20 gems twice in a row in most situations.

Unexpected wrote:
good only against zero cost decks in this form
maybe " If matching card cost is <20 next card will be Wraith" will be better idea

The condition could be anything such as matching card is common, cost <X, or zero cost. Whatever everyone thinks is best.