Yup - its kind of neat.
Its a medium swarm that can occupy the same space as another creature and that other creatures (friend or foe) can move through, but it counts as difficult terrain. It also can't be pushed pulled or slid.
It has a damaging aura, which does more damage for each additional rot grub aura the creature is in. It has no basic attack unless it has formed a zombie (which has its own profile and fairly typical zombie type attacks) - when the zombie is reduced to 0 HPs, it "creates" a rot grub swarm next to it.
The attack it does have causes ongoing damage and deals progressively more damage for each failed save.
Oddly enough, only the zombie form can give a target rot grub infestation (a fairly nasty desease that, if fatal, causes the creature to become a rot grub zombie.
That highest level version also has some attacks that immobilize or pull the target (its a sentient version of the swarm and has some psychic abilities).
Its quite interesting, really - I had just created my own version of a zombie creating worm -
http://www.enworld.org/forum/4e-fan...277397-plaguegrub-hydra-plaguegrub-swarm.html - and its intesting both what we did similarly and what we did differently... I'll likely be modifying mine to incorporate some of the things in the Rot Grub profile.