Effect updates and description changes can be seen here

Poison is a Perseverance deck specific effect card. It allows you to give KR to one monster (including your own monsters) and then draw a card. Drawing a card was added as an ability during Beta 5.2 as most well designed Perseverance decks revolved around using the various effects cards to gain an advantage, often running out of actual monsters in hand because of it.

As of Beta 9.3, it also adds another Poison to your deck, so you never run out of cards and can more eaily use it late-game where you're more likely to draw it over and over in the same turn, making it a good combination not just with Termination, but with Contamination as well. (Though, it makes you draw a card when you use it, so it can't be used to not take damage from having no cards left.)

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.