I always assumed that the Celestial Emissary was something created by the Church to allow them to make contact with the stars; the successful end result of Maria's experiments which also produced the Living Failures, things that resemble twisted versions of the Emissary itself. It's name can be taken one of two ways, either its an Emissary [i]from[/i] the cosmos or its an Emissary [i]to[/i] the cosmos, and I always assumed the latter; we know that the Healing Church made attempts to make contact with the cosmos, which failed as per the description of A Call Beyond, and we know that they were trying to create something in the Research Hall that also failed but which resulted in the patients with enlarged heads and the Living Failures. I think the Celestial Emissary is what they were trying to create and I think they were trying to make contact with the other Great Ones.

The fact that you get the Make Contact gesture from the Orphanage near where you fight the Emissary and the fact you can use this gesture on the Brain, a Great One, kind of supports this.

As for Great Ones not being able to die outside of a dream? How does Kos fit into that?