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. 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 (unless your opponent uses Kindness, as their healing effect goes before your damaging effect).