I didn't touch this for a while but today I felt like bumping it.
I added two new version of the card in the OP:
version 1 : All arachnids gain +1|+0. Inflict 1 poison per arachnid in play to the owner of this card when discarded.
version 2 : All arachnids gain +1|+0 (do not stack). Inflict 1 poison per arachnid in play to the owner of this card when discarded.
version 3 : All arachnids gain +1|+0 for one turn. Inflict 1 poison per arachnid in play to the owner of this card when discarded.
The first one can be abusable indeed. The problem is that you could generate up to 8 copies of the buff per use of nightmare. It means up to 6x8x+1=+48 per arachnid. sic
The second one overcomes the problem as the effect doesn't stack the arachnids can only go to +1|+0 and this until they die. The cost of 3 quanta is here the only effect that forbid the opponent to play all the cards at once.
The third version also overcomes the problem. Here the effect stacks. The more cards the opponents plays, the stronger the buff is. However, tis only last for one turn. It the version I prefer.
What do you guys think?