baldurs-gate-3
In the Abandoned Refuge, there is a loose rock next to a set of double iron doors at coordinates X: -596 Y: 423. The rock is glowing blue with a white light coming out. If I try to interact with the rock it says it can't be added to my inventory.
Why is this loose rock glowing? Is there something significant about it?
While to my knowledge, it isn't perfectly spelled out in-game, a reasonable hypothesis follows (basically this whole Answer is major spoilers):
The Chosen of the Dead Three come up with a scheme to steal an ancient Netherese artifact (The Crown of Krasus) and use it to dominate an Elder Brain (aka, the Netherbrain) to do their bidding. They do so. The Nether Brain does not like being magically enslaved, and begins to plot an escape.
The Emperor was a mindflayer operating outside the purview of an Elder Brain (and, in fact, had spent literal years waiting for an opportunity to escape). When its organization ran up against Gortash's influence in Baldur's gate, its true nature is discerned and, thus revealed, Gortash and co. bring The Emperor under the Netherbrain's thrall.
The Netherbrain, after dominating The Emperor, realizes two things: First, The Emperor will go to great lengths to avoid being dominated and enthralled. Second, this is an opportunity, as it has already realized that if one of the netherstone wielders is slain, that will weaken its slavers' control and eventually allow it to escape the domination.
This allows The Netherbrain to conclude that The Emperor is likely to facilitate such a killing, in order to secure its own freedom (or, potentially, claim the Crown of Krasus for itself). One problem: The Chosen dominate the Netherbrain, and the Netherbrain dominates The Emperor. If the Chosen forbid it from acting against them, The Netherbrain's thralls are transitively bound to the same rules.
So The Netherbrain lets slip that the Astral Prism contains a power that allows one to resist the domination of an Elder Brain, and subtly insinuates that The Emperor would be the agent best suited towards retrieving it. The Chosen fall for the gambit, and The Emperor successfully acquires the Astral Prism.
This temporarily returns The Emperor to independence, but The Emperor is desperate for a more permanent solution. But The Emperor is but one monster -- they're still surrounded by Absolutist forces, and, it needs to hold onto the Astral Prism (made more difficult by the Orpheus' honorguard). In short, it needs help.
Remember - Illithids are intrinsically manipulative. [Cite: in game texts]. The best way it knows to "acquire help" is actually spelled "coerce".
So The Emperor makes its way to the recently captured and tadpoles as many as it can (remember, at this point all the other Mindflayers on the Nautiloid are still under The Netherbrain's control -- hence the illithid corpses in the opening cinematic).
Now that they have a "sword of Damocles" above their heads, all that's left to do is escape from the Nautiloid and begin puppeteering its new "allies". Since Gortash directed The Emperor to pilot the Nautiloid (as per his journals), this is actually quite easy -- allow the gith to catch up and down the ship, use its psionics to protect its newly tadpoled "friends" during the crash, and retreat into the Prism where it can suckle Orpheus' power to maintain the status quo.
So why Tav specifically? Convenience, really. Wrong place / wrong time. The Emperor needed someone who could fight against The Chosen and be manipulated into pursuing those goals. Tadpoling the cast allowed it to achieve both those ends.
One thing that doesn't fit nicely into the above understanding is how the Sharran artifact retrieval squad fits into things. Shadowheart claims that they steal the artifact from the gith, Gortash tasks the Emperor to do likewise, and both can't be true. Then again, Shadowheart's memories are all over the place, and a little memory manipulation seems perfectly within The Emperor's wheelhouse.
The other strange wrinkle is that if you are playing The Dark Urge, the opening cinematic scene isn't canon for you. Specifically, TDU is tadpoled before boarding the nautiloid.
Which leads to an open question: Perhaps the opening cinematic has been mildly retconned? The Emperor's eye color is wrong, for one, even if the cowl otherwise identifies it. Consider too, that the cinematic predates the game's final release by ~3 years, and that a full 3d cinematic like this isn't nearly as easy to change as writing or dialogue.
So in short: the cinematic isn't perfectly aligned with the implication, so it's also entirely possible that the premise of this question is false, and The Emperor is not the one who does Tav dirty. It's ambiguous, and there's yet to be anything to conclusively verify it, one way or the other.
In Faérun, remember that souls are "real" in the sense that they are an intangible quality that exists separate from the body. The soul goes to the Fugue plane and becomes tangible when it dies (usually), and directly powers divine entities via belief.
In that sense, the "person-ness" of a being is tied to the soul itself rather than the body, or even the brain. But that should not be taken to mean that a soul is required for autonomy. Most undead, for example, are just fine with a soulless existence¹. Benji the ghoul even keeps a steady job at the circus!
The opposite is true for outsiders like devils, beings made entirely of soulstuff. Remember: every² devil was promoted from a lemure, and lemures are created by torturing mortal souls until they morph into a devil.
In D&D, traditional resurrection spells work differently on both of these classes of creature. In many ways, the return to life involves entreating the soul back into the body -- so creatures who are just souls or just bodies aren't helped by a spell that has nothing to stuff (or stuff into).
Which brings us to Withers.
Illithids have no souls
This implies that the mindflayer who arises from ceremorphosis is not the same being from before the process started. It might have copies of your memories, and it might float about in something that was once your body, but if the soul is gone, it's no longer "you". (This is heavily implied by the Act 3 Dragon Boss vis-à-vis The Emperor)
You are correct that this would also mean no afterlife (at least for the illithid itself) and there's an implication that The Dead Three's plan to control the world is flawed because it would reduce the number of souls entering the afterlife (and no souls means no divine power).
As to the rest of your questions, they relate more to the nature of illithid life than the ramifications of being "soulless". Illithids are eusocial creatures who are entirely subservient to their respective Elder Brain (The Emperor and Omeleum are very much the exception). While individual mindflayers might occasionally be dominated by their elder brain, their ability to work independently for the good of the colony is the primary benefit they provide. None of this requires a soul.
Orpheus' protection seems twofold -- it prevents being psionically dominated by an elder brain, and it sort of "pins" the memories of newly ceromorphed illithids in place; as soon as Orpheus leaves, a Tav mindflayer begins thinking unsavory thoughts, and the game gives you a choice to either commit suicide³ (before you become unrecognizable to yourself) or resolve to hold fast to your personality (following in The Emperor's example) despite the risk.
1 - Vampires are weird. Like most things in D&D, exceptions to these general rules exist.
2 - Again, exceptions exist.
3 - Though, since you're a mindflayer, you kinda already did.
Best Answer
This glow is what you get when one of your characters manages a perception check - something is up with this rock! I vaguely recall this one at the bottom of a climb:
Some objects in the game can be moved to reveal more interesting things underneath them. You can drag & drop the object to a nearby location or have a character toss it - strength stat might matter. Higher tier spells like Telekinesis probably work too.
If you have no strong enough character in the party you could gamble and try something like a potion of giant strength, hoping whatever is underneath the rock is more valuable than the potion itself.