Base of idea: Death is indiscriminate and inevitable.
Idea:
Permanent
Cost 2-3
Each turn, both players gain one poison counter.
Or, if one each is too UP, it can be increased to two counters for the opponent and one for yourself, or 3:1 or 3:2 or 2:2. Whatever ratio works best.
Practical usage: Great in a Purify/Puffer/Chrys speed poison, and also has synergy with SoSac (removing the problem of under 12(10) damage per turn from opponent).
I still need help on what sort of cost would balance this idea. I thought about it in comparison to Chrysaora, Arsenic and Poison. Chrysaora gives an unblockable +1 poison damage per turn to the opponent, but is vulnerable to cheap CC and can't do anything the turn it comes out. Poison is an instant 2 poison, but doesn't stack. Arsenic I think is the closest to this card, as it is also affected by PC but is blocked by a fair number of shields (diss, dim, etc.) and yet does damage as well for an initial two quanta. Arsenic can't stack though, while this card can - up to six copies on your side of the field. I'm thinking of a 2-3 cost for this, but if your opinion differs or if you think the card is too similar to Arsenic/Chrysaora to be worth thinking about, please let me know. The upped version might give two poison to both players with a cost increase, but I'm not sure at all about that.