Tree-Men of M'bwa

Once human, the Tree-Men of M'bwa become fixed in place and slowly turn into a human-shaped tree.

In the RPG...

There are four stages of Tree-Men, with each stage progressively becoming less human over the course of 100 years. All of their stats are appended to the human's originals. The first three stages are really the only useful ones in-game, as by the 4th stage the Tree-Man has 0 INT, DEX, SAN, and no ability to even move its branches, so is essentially just a plain old tree. Though fixed in place, the first three stages can still somewhat move their branches to attack, though with very low DEX and accuracy. Tree-Men are created when the zombie-like M'bwa forces its tree-serum down a victim's throat. A victim can be rescued within the first week, though they will forever bear thick, woody skin, and root-like growths from their legs and feet.

In a video game...

Due to their limited abilities, I would probably only include Tree-Men as a creepy Easter Egg.

In a film...

Though I've never read it, Donald Wandrei's "The Tree-Men of M'bwa" sounds like it would make an interesting film.