Garnfellow said:
Are there other mitigating factors? (ie, its primary attack is nonlethal or something.)
It attacks at +12/+12 (1d8+12, 2 slams) w/pre-factored Power Attack (it would be +17/+17, 1d8+7 w/o PA).
It also has Cleave, Great Cleave, Hide & MOve Silently +20, Spot +20, AC 19, Fort +6, Ref +9, Will +12. And forces Con checks (not Fort, but Con), starting at DC 10 & increasing by 1 per round to everyone within 30 feet; failure means unconsciousness, drop to -1 hp the round after that (if still in the aura), and die the round after that (if still in the aura).
Maybe it is CR 8; I haven't playtested it to see. But about the only way it looks easily defeatable by 8th level characters is if they have wide open spaces and more maneuverability. Plop a drowned in regular-sized dungeon, and PCs will die.
Perhaps if
water breathing protects from its aura -- but I'm not sure it does. "All breathing creatures within 30 feet . . . are treated as if beneath water in terms of being able to breathe."
Still, even without the drowning aura, with fast healing 5, 150 hp, and those attacks, it looks like a tough CR 8. Worse AC & reach than a stone giant, but more hp and fast healing. Add in the drowning aura, and you'd better hope
water breathing helps, and that you've got access to it.
Oh, it's Enviroment is Any, so it's perfectly capable of turning up anywhere, not just conveniently located next to large bodies of water that might suggest to PCs that having
water breathing handy is a Good Idea (TM).
Edit: it's not CR 18, though; it's not that tough.
It isn't alone in the questionable CRs, though. The CR 7 arrow demon (full attack: +14/+14/+9/+9 for 2d6+6 each, doesn't draw AoOs for firing while threatened; 135 hp; AC 19) might be kind of shaky. Compare with a hellcat, for instance.
Then there's the CR 11 mivilorn, with its 325 hp, swallow whole, breath weapon, and charging bite (which lets it swallow hole on a charge w/o having to grapple). It's SR is low, though.
(IMO, though, they don't look wildly wrong; maybe just a couple of points.)