The most likely reasons are if it's sacrificed, if it's legendary and another legendary permanent with the same name is controlled by the same player, if it's a planeswalker and another planeswalker with the same subtype is controlled by the same player, if it's a creature with toughness 0 or less, or if it's an Aura that's either unattached or attached to something illegal
Eggman are both Universes Beyond commanders that give your opponent the villainous choice of discarding cards or granting you a sizeable benefit
Its deceivingly simple, and yet, quite good
Some of the comments on TCGPlayer help to explain the price
Successful blink decks have existed in Modern before, thanks largely to Soulherder
Youve probably heard the word before, but what exactly are they