I don't think the +2 damage for the upgrade is worth a cost increase here (total of just 6 more damage). If anything, that +1 cost is for the art change
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At first I was confused about the name, but now I see the theme. I like the duos it encourages (darkness, entropy), but my main worry is one thing: SoFo. I can see it already - someone posting a deck called SoFo Sappers. I'm not sure how OP it would be, since you still need X burrowed creatures, but it would still be a very powerful combination. I see why it fits in Earth, but adding this rivals Fire in terms of PC-CC.