Baldur’s Gate 3 – Reasons for Astarion’s Inability to Ascend

The first character is the character that you actually play, while the second character is their "Guardian." According to Rock Paper Shotgun:

Strangely, the Baldur's Gate 3 Guardian does not appear to be an actual person who you can actually meet in the open world. They also, seemingly, aren't a character that you can control at any point.

Additional sources:

• PC Gamer
• Prima Games
• Dexerto

This is merely how your Guardian chooses to appear to your character. In truth, the Guardian is actually a mind flayer called The Emperor.

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:

It is nothing important, just a little extra loot.

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.