There's the vanishing mechanic, usually found on enchantments like Out of Time or creatures like Dreamtide Whale, which forces you to sacrifice that permanent when it runs out of time counters
Khrn the Betrayer Kharn the Betrayer is high on this list, not because I think it makes for a good commander, but because its an exceptional addition to other Commander decks that I like
Steamflogger Boss Steamflogger Boss is another staple card that appears in discussions of cards in the game with very unusual abilities
while it is a strong effect, you have to get it to stick, and also live to have 5 mana, and buddy, thats not very common at the moment
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