The Necroids and Hiveminds were not things that one could connect into one whole before. However, the Necrohives change all of that with a workaround.
One of the great bits of feedback that Paradox reportedly got when the Necroids Species Pack was first released, was that there were a lot of people who wanted to play Necrophage hiveminds. This will be possible in the next version of Stellaris: Console Edition!
Normally, hiveminds cannot have non-hive-minded pops living in their empire. This essentially defeats the purpose of a Necrophage, as they normally don’t grow, but instead convert the bodies of the living into their own visage.
To get around this, Necrohives will allow you to keep non-hive-minded pops as livestock, and moreover, these livestock pops will be able to grow, and then be subsumed at the Chamber of Elevation and converted into the primary species.
Like normal Necrophages, Necrohives are also able to use the Necrophage Purge to speed things up. In a change that also applies to all other Hives as well, Necrohives can now purge Gestalt pops that are not of their primary species. Genocidal Hiveminds the Devouring Swarm, and Terravore, now can also choose the Necrophage Origin.
Necrohive livestock pops are also granted a special exception to be able to live on Hive worlds. It is almost as if a murderous hive-minded species if is going towards some sort of a social liberation movement.