Punishment is a Justicedeck specific effect card. It deals 3 damage to anything you want, bypassing Taunt. However, if the enemy has more HP than you, it will deal 4 damage instead.
This can be a good card to get rid of certain cards that can work against you, like Diamond Boy 1. You can also use it as some chip damage to monsters that your passive can finish off or when you're about to use Asgore.
You can also combo it with a Mettaton NEO, placing it and killing it right away with Punishment, making you deal 8 damage to everyone except yourself. Just make sure you do this when you don't have any valuble monsters on the board.
If the enemy has more HP, it will deal 4 damage, able to deal damage to Mad Dummy without the need of damage modifiers like Papyrus Statue, making this the cheapest way to kill Mad Dummy.