Previous Versions can be found here.
Worsening is a Perseverance deck specific effect card.
It deals 2 DMG to a monster of your choice, including your own, and deals 2 DMG to the opponent. 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 an additional use, as when the monster you're using this on is under KR, the DMG to both the monster and the opponent is increased by 1 to 3, making it effectively a double Punishment in 1 card for the price of 1.