This ability triggers for each attacking non-human creature, which is why its so deadly
Plus, some cards (such as [c]Ox of Agonas[/c], which well see later down this list [spoiler!]), work better from your graveyard, so discarding them wont even feel like a downside
The most powerful decks of the past formats have been those capable of setting up a checkmate situation (Yata), swarming the field with monsters (Teledad, Lightsworn), or just executing a combo to leave the opponent without resources (Wind Up, Infinity w/Thrishula)
Getting a fairly-costed Crucible of Worlds on the side is enough for this card to see play
4 Scavenging Ooze Ooze That Just Keeps Growing 2 Mana Cost in Green Creature Scavenging Ooze is an infuriating creature to fight, going from a simple 2/2 upon its entrance to suddenly being a massive threat as long as its controller has mana and anyone in the game has a graveyard full of creatures
Like the other D23 cards, it is very limited, and the D23 Expo stamp makes it distinct from the same card which appears in The First Chapter