Prowling Horde

Skulk (This creature can't be blocked by creatures with greater power.)
Defile 3 — Whenever three cards are put into your graveyard from anywhere, if Prowling Horde isn't defiled, it becomes defiled and put two +1/+1 counters on it. As long as it's defiled, it loses skulk and has menace.
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Oh, they're here. You just can't see them.