Gwenom, Remorseless Blending deathtouch and lifelink with reckless value, Gwenom, Remorseless lets you play from the top of your library by paying life instead of mana
Mill per Card: 4 (8) Mill per Mana: 4 (8) This ticks two of the three boxes and is very close on the mill per card
While Martyr + [casthaven]Proclamation of Rebirth[/casthaven] is the engine to a more controlling build of Soul Sisters, here it is sacrifice this to turn on [casthaven]Serra Ascendant[/casthaven] or make target burn player cry. Its the most all-in card in this list and while it does synergize well with multiple cards in the deck, its the first thing I would cut if there were no budget considerations
Be among the first to experience the adventure and gameplay at a Prerelease event, taking place June 1622
Duress I card I associate both with old school formats like extended but also a card suitably balanced that it has seen countless reprints making it modern playable and periodically a standard card too
And as if that wasnt enough, there are also ultra-rare cards marked by unique colors or symbols (for example, an ultra-rare might have a white or gold star instead of a black one)