My theory on what's going on...
The place the brethren go when they die is not the spectral realm, or Raziel would have seen them at some point. So here is my theory on what's going on. Bear in mind it is probably wrong.
An unclaimed soul, upon death, abandons its vessel to wander the spectral realm. Dead vampires retain a link to their corpses, but normal souls wander the spectral realm until they can find another suitable vessel to inhabit. But, if Raziel or an archon devours a soul, enough energy to sustain them remains with them, while the soul is transported to a private sub dimension where the elder god can claim it. An extremely powerful soul can escape his clutches in the sub dimension, and become a rival for prey. Sluagh are souls crafty enough to escape his notice in the realms long enough that they can assume form. They absorb souls to fuel themselves, then use the excess to create more sluagh. Archons exist to hunt down sluagh and unclaimed souls. Sluagh help Melchiah because he can destroy tentacles where too many sluagh would be re-absorbed in combat. When the sluagh become powerful enough, they will pin the archons to the elder god room and manipulate the brethren to destroy his tentacles. Squiddy will starve. Or so they hope.
By the way, in the last chapter, Turel was out of character because he went slightly mad with solitude.
If you see any holes, let me know. Thank you for reading.