Akoum Hellkite

The dragon body is almost incidental to what this thing actually does, which is reward you for the same fetch-and-crack sequencing that any landfall payoff wants, then point the trigger at whatever needs to die. Flying and a 4/4 make it a clock; the landfall ping makes it a slow burn engine that scales with how greedy your manabase is. The Mountain clause is the lever: every red source that carries the Mountain land type upgrades the ping from one to two, so the card quietly asks you to skew your lands toward basics and dual lands that count as Mountains rather than colorless utility lands. That is the tension worth noticing. The payoff for committing to red sources is a steady stream of damage that can grind down a board of small creatures, finish off a planeswalker, or simply chip at a life total while the body attacks. A fetchland triggers the one-damage mode when it enters, then the Mountain it finds triggers the two-damage mode on its own entry, for three total off a single sculpted land drop. None of this is fast: at six mana it lands well after the early plays it would most want to punish. But as a top-end that keeps generating reach long after the creatures stop trading, it does something most six-drop dragons do not: it makes your land drops matter for the rest of the game.