Well, the plot is still a mess. I've decided it wasn't an imposter after all, but both of the zombie negotiators who were corrupted. By whom? An escaped Blood Angel. No name (not sure it has a name...the name may have been stripped from it). For offenses against the Ancient, it was sent to be a slave of Acharne, the Master of Meat and the Keeper of the Library of Forgotten Words. (Enough capitalized nouns yet?) It ran away, stealing a page from some evil codex or other, and took over the two aliens, planning to corrupt the treaty.
So yeah. I had fun inventing a few things. This week from the cliche bag: the Lost (yet Astonishingly Advanced) Empire --- I was going to need it eventually, so may as well start mentioning it here. Atlantis! Or not. It sank into Hell, not beneath the waves. More names stolen from defunct settings (Ichorian Empire, Minaxis Nochti - the City on the Lake of Night). Clearly this is where the obscure dangerous rituals originated.
Also, the Ridiculous Gnomish Artifact...they apparently made a huge invisible space station in high orbit while I wasn't looking. Damn gnomes! I plan to have one of my characters almost go there, but not quite. I hate gnomes, after all.
But anyway, after running scenes through my head all week, I finally sat down and started writing. Whee! I still don't have the ending worked out, but I'm hoping it will become more obvious to me as I write the story.