Yes. I'd go with contact poison (maybe ingested too). It seems like it should be pretty potent. I'm not sure I want it to be automatic death on a failed save, but maybe lots of Con damage. Like 1d12/1d12 maybe. Or multiple d6s in initial and secondary damage. That should be enough to kill a lot of PCs.
I wouldn't want it to be autodeath.
Most poisons in AD&D were death on a failed save but became ability damage in 3E, so I'd do the same here.
The original was save at -1 or die, which is more potent than a "normal" deadly venomous creature like a giant spider.
1d12/1d12 is fine by me. I'd be willing to go up to 2d6 Con/2d6 Con like a 3E Wyvern, which is a pretty good ballpark for an "extremely venomous" animal. The 2E AD&D giant poisonous snake and phase spider both have save at -2 or die poisons, but they only do d6/d6 Con and d8/d8 Con in 3E.
With the amount of HD and Con score of the Orm we don't need to worry about the save DC being too low.
Do we need to spell out exposure due to grappling, etc, or just leave to that to the GM, do you think? I guess most PCs are too small to grapple this thing anyway.
I'm leaning towards spelling it out, but it's not strictly necessary.
I also wondered whether the slime is a thin coating or covers its immediate area like the Mucus Cloud an aboleth can produce when in water.
The "trails of slime" in the original description suggests creatures close to it might risk exposure, but the special attacks says "save vs. poison at -1
if you touch it with bare flesh, or die", meaning it only poisons creatures who make direct contact with it.
Here are some possible models to consider:
Neotropical Toad
Poison (Ex): A neotropical toad’s poison functions as a contact, ingested, and injury poison.
Contact: Fort DC 14, initial damage stinging, burning, numbness in area touched for 1d6 hours. Poisoned legs force a subject to fall prone while at the same time reducing the subject's land speed to 5 feet. Poisoned arms or hands make it impossible for the subject to use objects or cast spells with somatic components. Venom touching the tongue make it impossible for the subject to use magic items requiring command words or cast spells with verbal components, and makes verbal communication impossible. Secondary damage none.
Ingested/Injury: Fort DC 14, initial damage nauseated for 1d6 hours, secondary damage death.
The save DCs are Constitution-based and include a +2 racial bonus.
Lumbrineris Worm
Venomous Bristles (Ex): Any creature that grapples a lumbrineris or hits it with a natural weapon is exposed to its poison.
Lumbrineris Bristle Venom: Contact, Fortitude DC 14, initial damage sickened for 1 minute, secondary damage 1d2 Dex and sickened for 1d10 minutes. The save DC is Constitution-based.
Aboleth
Mucus Cloud (Ex): An aboleth underwater surrounds itself with a viscous cloud of mucus roughly 1 foot thick. Any creature coming into contact with and inhaling this substance must succeed on a DC 19 Fortitude save or lose the ability to breathe air for the next 3 hours. An affected creature suffocates in 2d6 minutes if removed from the water. Renewed contact with the mucus cloud and failing another Fortitude save continues the effect for another 3 hours. The save DC is Constitution-based.