Mohrgs: freaky-looking undead murderers. While the undead type is great, the 14 RHD accompanying it aren't, and medium size isn't remarkable either. Their two natural attacks are a slam that deals unimpressive damage (and weirdly enough seems to be treated as if it's the mohrg sole weapon) and a tongue that makes paralyzing touch attacks. With opposable thumbs to go with it, that's not too bad.
Ability scores are decent. 21 strength and 19 dexterity are the only notable ones: the others are either nonexistent or average.
The first of a mohrg's other two special abilities are Improved Grab. This is bad because they're medium-sized beings considerably behind on BAB, and having to give up hands to grapple isn't helpful either.
The other is at the very least interesting: everything a mohrg kills rises as a mohrg-controlled zombie within days (hello, asterisk!), with no true limit on how many you can get in addition to the ability to ignore Animate Dead's HD caps. There's a number of questions this raises, though.
For one, can one zombify non-skeletal beings? Are undead gelatinous cubes possible? Undead lantern archons? Undead air elementals?
Secondly, what does the 'they do not possess any of the abilities they had in life' mean? Would a cloud giant zombie lose its Oversized Weapon ability, even if it'd normally retain it? Or does it refer to ability scores, and is a mohrg leaving immobile, insensate undead in its wake?
Weirdness aside, I don't think mohrgs would be overpowered even if they did get to keep their infinite spawn. Lots of undead HD ruin nearly any character concept for these guys.