The casting cost, the flavor text, the mechanics they all riff off of Nivs endless Curiosity (which to be clear synergizes incredibly with the first two versions of Niv, and is a straight up combo with Niv-Mizzet, Parun)
Marcy: This is a really, really strong card
Unlike in his original Commander deck, he fits well into here
Mages Contest You and target spells controller bid on life If you win, you lose life equal to your winning high bid and counter that spell If you lose the bidding war, the targeted spells controller loses life equal to their winning high bid, but the spell does not get countered Tibalts Trickery Counter target spell Randomly choose 1, 2 or 3, the spells controller mills that many cards Then the spells controller exiles cards from the top of their library until the exile a nonland card with a different name than the target spells name They can cast that spell without paying its mana cost Planar Chaos An enchantment that says you must flip a coin at the beginning of your upkeep If you lose the coin flip, you must sacrifice Planar Chaos Whenever a player casts a spell, that player flips a coin
Making someone else realize their Worst Fears is even better when you double it up with something extra evil like Counterlash or Spelltwine
