Hatchery Spider

Undergrowth's most ambitious payoff, and the one that most rewards understanding exactly where its dig fires on the stack. The reveal is a cast trigger, so it goes on the stack above the Spider itself and resolves independently: when that trigger resolves, you count the creature cards in your graveyard, flip that many off the top of your library, and put a green permanent of that mana value or less onto the battlefield. Because the ability is separate from the spell, countering the Spider in response does nothing to stop the free body; the trigger still resolves, the permanent still lands, and only the 5/7 goes to the graveyard. That is the entire trick, and it turns a clunky seven-drop into a genuine two-for-one gated on work you did earlier in the game. X does double duty: it sets both how deep you look and the ceiling on what you can put in, so the creatures you have already killed, discarded, or traded off simultaneously widen the search and raise the mana value you can cheat out. This is not recursion; the graveyard is only a counter, never a source, and the prize comes from the library instead. The leftover reveals scatter to the bottom in a random order, so an empty yard makes the trigger a whiff. What resolves is a reach wall that blocks flyers all day, but the green permanent stapled to the front is the reason to cast it.