1 HD a turn! That's ridiculous! We should make it 1 HD a day or hour. Something like...
Communal Immortality (Su): In order to destroy the horde, all its members must be slain, or it can rejuvenate. So long as a single horde entity remains alive, it can create new bodies or increase its own power at a rate of 1 HD a (time period) until the size of the previous horde is reached.
Communal Reliance (Su): The consciousness of the horde is communal, and leaves any member separated by too much distance. Any horde entity that travels more than 100 miles from the farthest other horde entity in its (horde?) is stricken as per a feeblemind spell, and dies in 1d10 days. Since the horde can communicate with each other and know of this weakness, this will generally only occur if a horde entity is forcibly moved.
I'm still not too keen on the phrasing of communal reliance. Thoughts?