Effect updates and description changes can be seen here.
Poison is one of Perseverance's original Common Spells. It allows you to deal 1 DMG to a monster of your choice and give it KR. If the target is already under KR, it will take 1 DMG and you will draw 2 cards (much like Blitz Quiz's old Zap Cannon). Keep in mind that if it wasn't under KR, you won't draw a card at all, making it quite different from most of its versions.
Drawing a card was added as an effect during Beta 5.2 as most well designed Perseverance decks revolved around using the various spells to gain an advantage, often running out of actual monsters in hand because of it.
Poison is a cheap setup card for Termination, which combined is a 6G single target kill card (much like Pollutant Gas and Contamination, which together kill the entire enemy board for 10G).
The effects of KR changed a certain number of times. But as of Beta 6.8, it does 1 damage to each KR'd monster at the start of their turn, and is able to kill it.
Card Skins[]
Karmic Retribution
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