15 RHD: so I'm kind of expecting this to be crap already. Let's see about that.
Aside from the plant HD, the twist has a considerable amount of strength, some natural armor, 10 ft. movement, DR 10/slashing, fire vulnerability, and three 2d8 slams (which the book has the audacity to call 'powerful').
The special abilities are a mixed bag. Death Curse is basically useless because by the time it has effect you're not playing a night twist anymore (the effect itself is pretty cool, though). Unholy Grace is a good ability that's only made better by +10 racial charisma.
Despair Song is quite interesting, being a large-range Crushing Despair effect that also forces affected creatures to move closer. Sadly, it's kind of limited by apparently not sparing allies, being limited in when it can be used, and not preventing affected foes from just attacking you once they've gotten to you.
Finally there's Wind Blast (free-action all-around Gust of Wind) which seems to have some utility in preventing medium creatures from approaching you, at least. Anything bigger than that can completely ignore the effect, though.
SLAs are okay. 3/day Deeper Darkness, Entangle, Blight, Fear, and Phantasmal Killer aren't the worst of spells, though there are a lot of creatures immune to all but the first two.
In the end, the night twist simply doesn't have enough features to make up for its large number of RHD. -0 LA, with a very slight tendency to +0.