Abhorrent Overlord

The clearest reward-and-cost engine devotion ever produced. The enters trigger pays out in flying Harpies scaled to your black pips, which means the demon's value isn't fixed at the rate on its body: a board already crowded with black permanents turns one cast into a flock. Even with nothing else in play, the Overlord's own guarantees two devotion, so the floor is two Harpies; the ceiling is whatever black symbols you stacked before you got here. That swing is the whole tension. The card asks you to spend the early game accumulating black mana symbols so the seven-drop arrives as a payoff rather than a clunky finisher. The upkeep sacrifice is the toll: every turn you keep the Overlord, you feed it a creature, and the Harpies it made are the obvious fodder. So the trigger that builds your army also builds the supply you'll be dismantling, turn by turn, which gives the card a self-consuming clock that punishes hesitation. Sit on a wide board and the sacrifice barely registers; let the Harpies thin out and you start cannibalizing the creatures you actually wanted. It belongs to the era when devotion turned every black mana symbol in the mana costs of your permanents into a resource to be counted and hoarded, and this is one of the most literal expressions of that idea: a finisher whose explosiveness is a direct function of how committed to the color you already were when you cast it.