I was off on the latter, but correct on the former ;)
Thanks.
Its got more HP than most CR 15's. But this thing can disapear into the woods and be awfully hard to find, along with having a 20% miss-chance. And this is assuming the players can make the save to even approach the thing.I would suggest raising the HP a little, since a party of level 15 would most certainly have magic weapons that can hit incorporeal enemies. If not the HP, then perhaps the AC.