The Green Heart on the Altar...

It's the same green stone found in the story [u]At The Mountains Of Madness[/u] by H.P. Lovecraft. Guillermo del Toro dropped working on an adaptation of ATMOM when he found out the storyline of Prometheus was almost the same. Which means: The Xenomorphs and the Engineers could be locked in an epic celestial struggle much like the Elder Gods conflicts with the Star-spawn of Cthulhu. Sounds like it was added CGI in post-production anyway. It wasn't in the original cut.

I think it's the Proto-Xenos. Engeeners one day found this cristal with an entity inside, traped in the cristal like mosquitoes are trapped in amber in our planet. So it's a kind of "dinosor DNA" for them. They exploit it and found it's an incredible predator. They then try to refine its DNA to exploit it into the BioWeapon we have discussed so far. They seem to have difficulties not to raise him, but to make him to reproduce itself (Because Engeeners don't have any gender ?). I suppose the eggs we see in Alien 1 are ancient, and the Boxes we see near the cristal in Prometheus are a more recent refined type of Xenos. That explain the Deacon : they where working on Xenos for a long time. A little bit like we admire old weapons and put them on our walls... For them, it's an old subject of work/admiration/satisfaction. We have swords in our blasons and walls. Why would'nt they have Xenos ?

Also discussed in my Engineer/Xeno connection thread.

QFrench: you know, what you say makes all the sense in the world!!!! I made the thread just for kicks, but you assumption is really interesting and logical.

Prenihility: Sorry mate, it seems we thought of the same thing at the same time. [but to be truthful, I posted first. MUAH-AH-AHHHH]