I love to use Knowledge skill rolls in my games! The players get a kick out of learning something new from a decent roll on a high skill. But, I will be honest, depending on how esoteric something is, an 18 might not be good enough. For unusual undead, my DC's are usually above 20.
With 3.5, there are now rules in there that formalize the DC. IMC, that will be a good starting point for most monsters. For the really rare creatures though, I will probably add a modifier. I don't have the stats for the Scion of Kyuss, but if this had been a Spawn of Kyuss, with 4 HD, you should have gotten something a little more specific. But, the question then comes down to what does the character know?
does the character know that a successful Remove Curse, or Remove Disease will turn the Spawn into a normal Zombie? That the slam attack can spread a supernatural disease that drains Wisdom and Con? That the worms will eat your brain in a matter of minutes? That the Spawn is harder to turn than normal?
It's a judgement call. I am not quite willing to say that your DM screwed up. Maybe the DC to know anything about a Scion of Kyuss is higher than 18, for whatever reason?
If his idea of providing pertinent information was the description of worms, then I would say that was a bad call. There is certainly more pertinent information then the observation of worms.
Now, if you didn't reach the DC, then it would have been more helpful to say "You don't recall anything special about these. They look like Zombies, but zombies don't normally cause your buddies to flee in abject terror. There also seems to be a higher quantity of worms crawling on their bodies than for normal zombies." Basically, I would have played it up that even those these things look like zombies, there are enough clues that the character knows something is odd about them, even if he can't clue into the exact details.
Now, I will pretend that this is a Spawn of Kyuss in my campaign with 3.5 rules. The DC to know about a 4 HD undead would be 14. Because they are exceedingly rare in my game, I would probably bump that up to a 19. Your roll of an 18 would tempt me to drop my modified DC. I would probably resist the tempation. But, I would give you a circumstance modifier that as soon as a worm jumped out and started burrowing, you would recall details! For me, that would help keep up the drama and keep the fear of death alive, without dooming the characters totally. Now, if the Knowledge roll had been lower, it would have been bad! Of course, that is just my style. Some people wouldn't apply a modifier because a Spawn of Kyuss might be more common in their game, or because they have a different DM style.
