(Sorry, this'll be more succinct than usual)
I rather like this. I like when the base game is expanded in a way that works, and the NE just don't have enough "alternative Treant Buildings" in the archives.
I really like the 3-head thing; smacks of that one judge in the early Transformers movie. However:
I get it that he is supposed to be a Treant Janus of sorts but it actualy took me a while to notice the side faces of his. Maybe you could find a way to better highlight those artistic features of the model?
This very much so. Bring it out! It's the best part of the model!
I like the mesh & textures (very much fits in-game), I love the SFX (very interesting)... The name is cool ("Ancient of Lies"), and all that.
I have only one issue, and I may be wrong: but it appears like there's a whole lot of green & not a lot of other things. Like, you have the horns & the TC in a few places, and the skull-knee-caps are neat... But there's just so much leafy green (& and especially on the back with the cloak) it's really... bleh.
On that note, maxing out the Janus part might solve this problem. That and/or adding something to the horns or the shoulders/back/top-of-cape might help.
On another note, why is it a "Dark Ancient"? It really doesn't look that evil to me (or is this just more of your naming shenanigans?).