I made this card a few months back as a gift for a friend of mine. I wanted to make something that'd tickle his sense of humor with both the mechanics and the flavor text, that also had the capacity to grow with his character. And so, naturally, I made a knife that makes people shit themselves to death.
Whether you handle the long-term effects of the knife as poisoning or disease is up to you. The disease angle fits the loose idea of it causing some hellacious intersection of dysentery and sepsis, but treating it as poison may work better for your party balance.
I've found that the best time to ask for the CON check is immediate after a long rest, only letting the rest reduce their exhaustion level if they pass both the current day's check and the previous day's check.