So it's a
themed Shard of Void with a neurotoxin-esque effect that becomes more powerful as the opponent plays more permanents...
It's an interesting concept, I'll give you that. But it'd be kinda weak in play. Max HP reduction is pretty weak in the current meta, unless the focus of the deck IS the HP reduction.. Having said that, this could make for a very interesting
duo stall, so for that I give it a thumbs up.
Here's my issue though: It's very, very specific as to what causes an increase in Pox counters. Why is this? In it's current state, it's very weak because
1. Creatures tend to be the focus of most decks (see speedbows, immobows, -tals, etc.)
2. Permanents are not typically played in abundance.
Including pillars/pends could have helped make up for this, but...you specifically exlude them. Why? Is there a thematic reason that I don't know of? (for what it's worth, I have no idea what "senescent" means, nor do I know the thematic inspiration for this card)
I do feel that it would be wrong to make it a pure neurotoxin effect, but with HP reduc., because that could overshadow neurotoxin. Dealing 1 damage is not as powerful as permanently removing 1 hit point. Both have the same gain, but the latter is a hard counter against anything with healing.
EDIT: While writing this, I had thought that only non-pillar permanents would cause a counter increase. Didn't realise that this would also apply to spell/alchemy cards.