For example, McDonalds occasionally releases them in their Happy Meals, like their 2023 Scarlet & Violet promo release
Callidus Assassin replaces whatever it copies, which is like stealing opponents creatures
If you need a huge discount to afford this card, HP copies sell for as little as $35.
Actually, given the popularity of DuplicantDuplicant in blink decks like Brago, King EternalBrago, King Eternal or Aminatou, the FateshifterAminatou, the Fateshifter, it's clear this enters-the-battlefield effect is pretty appealing to a ton of players out there
Temporal Fissure might not literally win the game the way Tendrils of Agony or Grapeshot do, but it certainly kills your opponents enthusiasm and desire to keep playing
I would heartily recommend the experience to anyone, and particularly those players who might be feeling a bit burned out on regular games of Commander