Themberchaud

The entry trigger is a symmetrical blast keyed to your own Mountain count, and that scaling is the whole reason the card exists as more than a vanilla body: in a mana base packed with Mountains, arrival becomes a Pyroclasm-scaled sweep that also burns every player's life total. The relevant count is any land with the Mountain subtype (basics, shocklands, triomes all count), so the ceiling rises with how deep you commit to red sources. Because the damage hits each other creature, the dragon himself is spared, but the resolution is genuinely two-sided: you take X to the face right alongside your opponents, and your own ground creatures die to it just as theirs do. That is the real tension of any board-clear-on-a-body, and this one leans into it by pointing the drawback at the whole table, yourself included, rather than pretending to be a clean one-sided wrath. The exert clause is where the design shows its second gear: a five-power trampler that skips ground blockers by taking to the air, at the price of staying tapped through your next untap step. That is a defensive turn surrendered, not a free keyword, so exerting is a decision to commit to the race. Both of the card's most powerful lines cost you something on the way in: the sweep costs life and your own board, the alpha strike costs a turn of untapping.