Which units would you like to replace/add?

A friend of mine suggested a unit (undead tier 3), and I haven't seen anything like it in any other undead faction. At first, it seemed like nonsense, but if you think about it, it's very original and makes sense.
It's called "Rot." It's a dead, rotting tree that the undead have controlled (like everything else that's dead). Stats:

Attack---------6
Defense-------6
Damage----------4-6
HP------------16
Speed------4
Initiative---8

Ability - Crumble. It can crumble instead of moving (it's a rotting tree) 6 times in battle. Its crumbled form is invulnerable to everything, and no one can step on it. If there's an enemy ranged unit nearby, it can attack, but only with a penalty at any range (the crumbled form gets in the way).

Added 6 minutes later

But they're not very useful.

They weaken the enemy well, absorb return attacks (they have a lot of HP), and protect the skeletal archers well. Zombies aren't that easy to kill with a couple of hits, and the thousand and a half skeletons I've gathered will shoot at close range, and that won't be pleasant.