In most situations, they dont do anything at all
It's how the app has been monetized, with the developers hoping that players will find it hard to resist some of those rarer cards, buying up packs in the hope that a few will appear
Its especially powerful with evoke options that sacrifice themselves, and you can even set up some loops with Kodama of the West Tree or Perilous Forays for infinite endgames
Because of that, though, one of the bigger predominant decks of Pauper is Mono Red Madness
Roach Invasion When Shiny Black "C" is in your Graveyard, Banish it to destroy a Synchro monster as soon as it hits the field
The card types that can appear on cards you reveal with Portent of Calamity are artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery