Ferret said:
Looks pretty good. I like the Fiendish Rat Swarm, even though it's not quite "by the book" (can't apply Fiendish template to Swarm type, but who cares?) Looks like you forgot the DR 5/magic though.
Were you planning on doing anything with that "multitude of insects that crawl from it with trapped hunger" inside the barrels? You could always add an extra CR 1 insect swarm (based on Spider Swarm) for flavor. Even if the PC's don't open the barrels, any crit miss adjacent to the barrels could burst the rotten wood and release the swarm.
You should also clarify exactly how close they can step to the crates before the rats attack, and probably add some sort of Spot check to notice the rats. Where is the derro zombie to begin with, and why doesn't the party see him - inside one of the crates?
Remove the line "There is nothing else of interest in the room" from the description. At best it would be "You don't
see...", but then again that's rather redundant as well.
OOPS... looks you forgot to apply a bunch of the zombie template features. Including ability adjustments: "
A zombie’s Strength increases by +2, its Dexterity decreases by 2, it has no Constitution or Intelligence score, its Wisdom changes to 10, and its Charisma changes to 1.". Saves need to be recalculated as well (poor Ref save).
Mind if I as why a zombie Derro? Doesn't seem like a great combo, because the Derro loses all his cool stuff (poison use, sneak attack, spell-like abilities). Actually, he should probably lose his Madness SQ as well, since he's now mindless anyway (plus the Zombie template fixes his CHA and WIS stats).