The spell description (below) speaks of poisonous effects and you do make a saving throw vs poison so it's kind of misleading if poison immunity does not affect it.
The Stinking Cloud spell creates a billowing mass of nauseous vapors up to 30 yards away from the wizard. Any creature caught within the cloud must roll a successful saving throw vs. poison or be reeling and unable to attack because of nausea for 1d4 + 1 rounds after leaving the cloud. Those who make successful saving throws can leave the cloud without suffering any ill effects, although those remaining in the cloud must continue to save each round. These poisonous effects can be slowed or neutralized by appropriate magic.
You'll also experience your Cavalier dying in a fit of toxic gas with the Cloudkill spell too if he fails his save.
When it comes to non-magical and natural poison, you're good. These will be experienced on the dipped bolts and arrows of one's enemy, dripping fangs of poisonous spiders and raking claws of ettercaps.
Cavaliers are immune to being poisoned, not poison damage.
Really? I had the impression due to the way the ability shows up on your character record that it SPECIFICALLY protected against damage from poison, but not special effects (incapacitation, instant death, etc.).
I believe the only thing they're immune to is damage caused by poison, but secondary effects like falling asleep or instant death on failed saves still applies.
Game mechanics, that's all. Cloudkill applies damage, effect 12, that simply happens to be listed as "type = poison"; meanwhile, the cavalier receives immunity to effect opcode = 25 (poison). If their other applied effect of "100% poison resistance" doesn't give them immunity to Cloudkill, then just update CLABPA02 or spcl222.spl to include "immunity to spell" targeting cloudkill specifically. Stinking Cloud is another matter altogether since it uses opcode 39 for "sleep".
Pencil-and-paper rules are always more flexible than computer-based rules, obviously.
But the thing is that Marek's poison isn't even a major plot line. It's just a quest. Even if that quest is removed, it doesn't really affect anything.
Comments
The Stinking Cloud spell creates a billowing mass of nauseous vapors up to 30 yards away from
the wizard. Any creature caught within the cloud must roll a successful saving throw vs. poison
or be reeling and unable to attack because of nausea for 1d4 + 1 rounds after leaving the cloud.
Those who make successful saving throws can leave the cloud without suffering any ill effects,
although those remaining in the cloud must continue to save each round. These poisonous
effects can be slowed or neutralized by appropriate magic.
How can you take poison damage if you are totally immune to the cause of the damage?
If you are immune to poison, you should be able to sprinkle it on all your food and then ask for seconds.
Stinking Cloud is another matter altogether since it uses opcode 39 for "sleep".
Pencil-and-paper rules are always more flexible than computer-based rules, obviously.