Cleon
Legend
You're really on a roll. I think that looks pretty good. They're reasonably comparable to the Colossal monstrous spider. They have fewer hp and do less damage, but the cloud and death throes make up for most of that.
Actually, I think we didn't settle on the choking cloud and death throes yet! So maybe it's too soon to look at CR. Are we keeping stunned or going with something less traumatic? As for Con damage, does 2d6 sound good? How much worse should the death throes be?
Oh right, I was thinking something like:
Choking Cloud (Ex): Whenever a greater dust spider takes damage, it releases a cloud of dust in a 20 foot spread. It can also release a cloud of dust as a Move action. All creatures within the cloud must succeed on a DC 23 Fortitude save or be sickened for 1d4+1 rounds and take 1d6 Constitution damage from violent sneezing and choking. Those that save are merely dazzled for 1 round. The save DC is Constitution-based.
Death Throes (Ex): When a greater dust spider dies, it explodes in a 40 foot spread of deadly dust. All creatures within the cloud must succeed on a DC 23 Fortitude save or be nauseated for 2d6 rounds and take 2d6 Constitution damage from violent sneezing and chocking, those that save are merely sickened for 1d4 rounds. One minute later, all creatures that were covered by the dust must succeed on another DC23 Fortitude save or take an additional 2d6 Constitution damage. The save DC is Constitution-based.
I was tempted to give the Death Throes a racial bonus on the DC but decided that's nasty enough.