It can be planar bounded by a wizard of its level! Clearly it must be -0!

Of course we are not going to compare the Cauchemar to full casters. Even though its most defining feature is its incredible SLAs, it would function much more efficiently as a martial, or a totemist. Its smoke is better if it can be stacked several times in a row, and its stats are extremely combat oriented. Let's have a look:

• 15 Outsider HD. Another evolved monster who is doomed to -0 by the number of HD, but contrary to normal, the Cauchemar is based on an absolutely broken monster for its number of HD, and is an outsider, so there might be hope for a more reasonable negative LA. Outsider HD are really good, of course, but they're nowhere near class levels, especially at high level like the Cauchemar is.
• +20 Str, +4 Dex, +14 Con, +6 Int, +2 Wis, +2 Cha. +16 Natural Armor. That's... Not that good. Don't get me wrong, those are incredible bonuses, especially for a martial with even an Int bonus, but for 15 RHD, that's much lower than a lot of other monsters, especially outsiders.
• No ability to wield weapons, but 2 good hooves attacks (pretty rare kind of natural attack), and the all-important bite that will let them use a mouth weapon. That is good. Not incredible, just good.
• A very good (Su) ability that probably should be (Ext) with Smoke, a cloud useable by a free action who debuffs in a cone and grants the Cauchemar concealment in that direction. That is extremely useful in combat and would be too to approach enemies without getting blasted from afar, that is if the Cauchemar didn't have...
• Astral Projection and Etherealness at will. My gods those are strong. Etherealness is very good by itself. It allows you to pass through walls, not be affected by anything for 20 minutes, infiltrate anything with the whole group, make a swift action attack before the enemy can act by dismissing Etherealness behind their back...
But the whole kicker here is Astral Projection. One of the most broken spells in the game. Planeshift, Clone, arguably even Fragmented Phylactery, all for the whole group, without material component, by a standard action. Plus a whole lot of abuse of limited-use items, or even of non-limited use items, if your DM allows it. In the original thread, Inevitability didn't consider Astral Projection when judging both forms of the nightmare, mostly because Astral Projection would be utterly ridiculous at ECL 8, but I don't think it is that unrateable around ECL 15, only 2 levels before wizards get it. Still a 9th level spell, but we're at the level where there is so many super strong spells everywhere that this one doesn't make the whole monster broken.

So, a pretty good fighter with excellent utility, and the ability to enter a lot of prestige classes. What does that make the Cauchemar? Really, with astral projection, etherealness and smoke, I don't think it deserves a negative LA. You'll probably object, but I'm gonna go with 15 RHD and LA +0 as a first draft. Feel free to object. Without Astral Projection, it is way less interesting, but is still a good brawler with interesting abilities and outsider ability scores (low outsider, but still). I think I would have given it 11 RHD and LA -2, since full BAB RHD are less valuable at high level than they are lower.

This was a weird one to rate, and I can only imagine the difficulty for Inevitability to rate the usual Nightmare (seriously, who names the evolved version of a monster with the translated form of the name of that monster?). I still hope I did an okay job, and we will continue with very-high powered monsters next time with the three flavors of Night Shade!