Previous Versions can be found here.

Worsening is a Perseverance deck specific effect card.

It deals 2 damage to a monster of your choice, including your own. This makes Mettaton NEO a good monster to add to your deck, as you can use this to kill it instantly. (This works the same way as with Justice's Punishment.)

But this card has a much better use, as when the monster you're using this on is under KR, it does 5 damage instead, able to kill monsters such as Mad Dummy. And when it kills, any damage it did more than needed will be dealt to the enemy directly, similar to Casual Undyne.

You can also use it on monsters with 3 HP or less, as they will then die either with or without Perseverance's passive effect.