Look up the creature Type descriptions in the SRD. Remember, there were only so many pages in the Conan rulebook and as an OGL ruleset it would have been foolish to waste space reprinting something like the Type descriptions, but nothing in the Conan rulebook suggests we should disregard them either.
Undead are immune to mind-affecting spells and effects, Swarms have the Vermin type and likewise are immune to mind-affecting effects.
Hope that helps.