From the Monster Manual...
QuoteSoft, squelching footfalls and a strange, moldy odor indicate that some creature is nearby—but nothing can be seen.
[INDENT]This creature looks like a brown and greenish-brown mass with a cluster of nodules atop the main mass, though it is visible only when dead. A cluster of nodules atop the main mass serve as sensory organs. The creature feeds and attacks with a gaping aw lined with rows of teeth. Four stumpy legs support the creature and allow it to move about. This ambulatory fungus is naturally invisible, making it a feared predator among subterranean inhabitants.[/INDENT]
COMBAT
[INDENT]A phantom fungus usually roams quietly, hunting for prey. It attacks lone individuals almost anywhere, but when tackling groups it prefers an open space where it has a better chance.
Greater Invisibility (Su): This ability is constant, allowing a phantom fungus to remain invisible even when attacking. It works like greater invisibility (caster level 12th) and lasts as long as the phantom fungus is alive. This ability is not subject to the invisibility purge spell. A phantom fungus becomes visible 1 minute after it is killed.
Skills: A phantom fungus has a +5 racial bonus on Move Silently checks.[/INDENT]
Considering all the Underdark theme and our spreading of moss, mold and fungi monsters... this could be a nice companion to the eyeless spider in the Shroomstalk Forest.
Permanent invisibility? That's just cruel.
If it changed to invis every 5ish seconds like the sorc perk, I think it would be fun.
That perk is invis every 10 rounds. That would be intervals of one whole minute.
Also when out of combat I think? Permanent invisibility isn't too horrible I think. It's like 50% concealment and nobody complains that ghosts are missed half the time. Once you start getting beat on by it you'll realize something is there.
Though an alternative could be to pop some DR on it and give it a lot of Invisibility spell casts like how Will-o-wisps work, or improved invisibility since -technically- in pnp the way improved invis works is attacking does not break invisibility, which is why it keeps providing the 50% concealment in nwn. I've known other pservers where they fix this so it provides no concealment but you stay invisible while attacking. Cool stuff, as long as the enemy can't cast see invisibility! Nice monster find!
I would like to also recommend the possiblity of a gas spore monster running around. Fake beholders of toxic doom are neat.
Improved Invisibility could be what we're looking for with added transparency to them, maybe?