I've been trying to think about how you show mind control without having a visual cue that it is happening.
The obvious answer is that someone suddenly changes their behavior and acts weirdly.
'These are not the droids you are looking for.'
Here, you can't even see the stormtrooper's face, so there's no clues in facial expression and no special effects to show he's been affected. Instead, the clues are all in Obi-wan's behavior - the hand motion, the careful, expectant tone of voice. And of course the fact that the person who stopped and questioned them simply repeats what he's been fed. Of course, you get an explanation after the fact in case there was any confusion as to what just happened.
The vampire Alucard checks into a hotel:
...and checks out.
In both scenes, they show him using some sort of mind-whammy on a person by showing that person's eyes turn red. But more significantly, the person becomes deliriously happy for no reason, eager to please, smiling, etc. It's rather obvious that the people affected are not acting rationally or making their own choices here.
In the case of the Spell of Bottomless Dread, we're not going to show the victims being happy or smiling - they should be terrified. So, the compulsion is much more of a push than a pull. They're being forced and can't resist, but not happy about it. It should come across more as possession, probably. But...we don't want to give the impression that Morgoth himself is present. It's more a....puppets on strings situation. He can pull the strings when he wants to. Showing someone completely ignoring/tuning out someone they care about and turning away, totally focused on the task Morgoth is telling them to do....that sort of thing. We might want to have the victims hear voices, I dunno.