Simon's already turned into the blood tree by the time the Eridians discover AT-5. They bring back what they think is a man, and everyone is all excited thinking Grace is about to have a companion and, y'know, the pretty common opening to most Bloodymary fics out there.
That's not fucking Simon.
Well, that's not Simon alive.
It's the equivalent of a still-life (iykyk). It's something the ocean constructed out of blood and whatever chunks of flesh were leftover from the sub exploding to try and remember Simon for itself with. Something not-quite nerve ghost, not-quite man, and not-quite monster. It's a man yet mangled. It's a monster yet never attacks. It's a phantom yet touches. Half-measured. A culmination of things Simon has always been scared of.
The Eridians bring back the fucking shadowman, and now Grace has to figure out what to do with it, if he can pry pieces of Simon out of its cold dead gaze, if he can learn about AT-5 through its chemistry, if he can show it enough care that it starts looking more man than creature, and, it's all morbid, so morbid, but Grace can't stop obsessing over how this thing even exists, because it's inherently paranormal, which shouldn't exist.
Oh, and I like to think it parrot-mimics things Simon has said in the past from time-to-time in the corner. Once in a blue moon, when it really wants to be noticed, just to startle the fuck out of Grace.
And the Eridians are spending a lot of their resources investigating the blood tree while they're at it. The "real" Simon is chilling out on AT-5 as a glorious, massive, flourishing organism that the science team is obsessing over. He's getting lots of attention over there.
I really think the shadowman is an underused concept in fiction. Gimme more.