It's always stated that the Sleepers are mindless and instinctive, just like classic garden–variety zombies, but I thought, what if this is not actually the case? If so, the Sleeper's apparent mindlessness would be simply a façade, covering up the deep intelligence within. After all, Avarrach had the technological prowess to manufacture nanobots which utilise flesh for creating more of themselves, so why not intelligence?
One of the ways artificial intelligence could have been implemented in the Undead is to have multiple central servers (players IRL), and radio transmitters which are powerful enough to send signals across vast distances and past numerous obstacles, through the inter–dimensional rifts. If the entire world of Avarrach had been taken over by the Virus, powerful supercomputers all over the world (e.g. military defense systems) would have been also assimilated. They would then be used to process large amounts of information to develop strategies, and facilitate the Undead in their movements. The hordes themselves would also have the ability to receive these signals.
TL;DR The Undead hordes form the 'body' of the Virus, and supercomputers resident to Avarrach, the hive mind.