The symbolism in Mike being chained to this specific rock…
Okay, so most people have already figured out that the rock is shaped like Mal’s head. This serves several purposes:
1. It is the first time that Mike really realizes that Mal has returned
2. It represents how Mike (and possibly his other personalities) are just prisoners to Mal
3. I know I already kind of said this, but it’s like a terrifying reminder that Mal is in control, and there’s nothing Mike can do (which is why he’s chained to the head, and it’s not just lying around)