I think it might be alike to the Khrave in that they are partially Warp-beings, a hive species that makes of a species and/or area a territory of sorts, and within that territory they form a sort of psychic bond or parasitism to the species that calls it home. From that bond they are able to produce dark mirrors of the species' own creations--that's why Sedna is called a "nemesis world", it's a false world based on being the iconic foe to it's prey, sort of like the principles in which daemons are produced by acts and symbols done by sapient beings. And the Kuiper Belt encircling Sol is the means by which they project their territory, locking around the cradle of humanity like rats in a cage. Sedna itself is probably the greatest indicator of the "bond" being formed with a species, and with it's destruction via psychic means the bond is extinguished and Sol freed--possibly literally if you take it to mean that they were precluding any sort of astropathic message or Warp jumps.
And it is why the Emperor needs to lead the assault, for in him is humanity's psychic potential embodied. To break the trap, the bond, requires not just power, but symbolism. Mighty symbolism.
