If there're skeletons, then I suggest a lich-like monster, which aside from also being pierce-immune (still boney :P ) can reanimate fallen skeletons.
In context, I also suggest this:
Name: Spinedragoon
Description (so you see what I have in mind, and the source of the name):
Once assembled (see below), looks a little like a bone wyrm. Before that, like a group of untypically synchronised skeletons, and before that it looks like a thin mist, if it is seen at all.
In its first cycle, it floats around searching for fallen skeletons, taking their forms into into itself.
This brings it to its second cycle, where it leads several formerly animated skeletons in search of more. It keeps them looking like skeletons in order to avoid attracting attention (as far as that goes for the walking dead, anyway) while still weak compared to its potential.
It enters its last cycle once it has found enough matter from which to create its corporeal manifestation, and after entering a brief trance-like state where the bone-matter asserts itself into place, it takes on a new form - that of a huge, serpentine, wyrm-like mass of bones, sometimes with a "rider" at the "shoulders", one looking much like a hellish, bone-armor clad knight, fused at the waist to the rest of the beast.
Although it continues to collect bones into itself for more power, this is the final basic form it takes.
Although the "spirit" is the main force behind this beat, the bone matter, besides being a platform of its power, is also a source of it - the more bones, the more powerful, conscious and sensorily aware it is. Therefore, while it may carry the bones, and these may encumbre it in the second cycle due to the "mental" strain induced by the act, by the third cycle the power gained outweighs that lost, and it carries itself much better than ever before.
It is important to note that once the spirit form begins binding to skeletons, this cannot be reversed, and if all of them are destroyed a second time, the spirit form will die.
Strength: [cycle] 1 - none; 2 - as per skeletons; 3 - superb
Dexterity: 1 - legendary; 2 and 3 - good/great
Speed: 1 - good; 2 - fair; 3 - great
Attack type: melee. 3rd form might have a ranged "unholy blast" type of attack, related to its excess power (so it possibly drains the Spinedragoon's bonus HPs, see below).
Special abilities: see above. I assume mass collected in the final form will add to HPs.
Behaviour: 1 and 2 - tries to stay out of trouble, and collect bones. The first cycle may be attackless, but the second can use skeletons as they are (though likely with better stats). If Liches are included, or any other animating being, it will try to make it look as if the rising skeletons are the other's work, to avoid needless attention. It may even stop collection temporarily if the raising being's killed.
I'm not sure how many skeletons would be needed for the final cycle - too few, and it will be overpowered, but too many and the skeleton band'll be difficult to manage realistically. Maybe an inbetween form could be added, where there are many skeletons included, but not enough for the last cycle.
3 - nemesis incarnate, acts directly and intelligently to slay the char.