Thaniir Ancient (Huge Fey Plant) - CR 25

The Uld Thaniir is the strange tree God of the Tal'Mir; a race of feral, plant infused aelwyn, similar to the Syra'Syth Aelwyn of Arbel'Verdaniss - though more savage and given to using poisons and torture to change natural creatures into horrific servants and guardians. The Tal'Miri Forests that are home to these creatures (which dwell in Synnok, a fey dimension of 13 sundered realms...ah, you don't need to know this do you); are a vast, wild realm of luminous Glymwoods, Fey Candles and other strange, magical trees, and as a result, they are scattered wildly.

To help them stay in contact with their deity (who resides deep in the Forest in the Court of Thornheart - Tal'Jonde), the Uld Thaniir sends fragments of his essence out, which create the vast Thaniir Ancients; Treant like beings that defend Tal'Mir settlements, and relay the word of their God to the people.

Art is not my work. Artists' credits on picture.
This, but with more animal skulls, blood and stained rune stones around it.

My PUG have managed, mercifully, to avoid contact with any real Tal'Mir entanglements (although that is about to change most likely), and so, have not been close to one of these things (which would annihilate them in a round or less).

In your own games, an Ancient would make a great end boss for adventures centred around dark druidic cults, aelwyn (elven) attacks or anything of a similar nature.