system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] What does the soul stone do? [Spoilers?]

We saw a demonstration of every single stone, except for the soul stone. 

Keeping in mind that they are different than the comic book gems in some ways, If a MCU villain/hero was able to get only the soul stone, then what are they capable of doing? Everyone assumed it was going to be Heimdall carrying it, but maybe he was given power by it in the past? I ask because this was the most protected stone, but it is never used. 

The mind stone was able to give consciousness and emotion to a robot, as well as control both of those of a living being. So where does the soul stone come in?
gpt
We see Red Skull greeting people by their names and the names of their parents, which is definitely a Soul Stone power. And we see Thanos talk with Gamora, presumably because her soul was trapped in it after she was sacrificed.

It probably lets you perceive who a person is, at their most fundamental level. The kind of knowledge only a best friend or close family member would have, not mind reading but a gut feeling about how they would react in a hypothetical situation. You could also change who a person is if you exerted its power, and a more subtle version of this would let you say or do things to easily manipulate someone.

And you could do this to yourself, although that's extremely risky. One common theory is that you could use it to make yourself worthy to wield Mjolnir. But then you'd only be able to use Mjolnir to do worthy things, so it seems difficult to abuse.

It could almost certainly be used to capture peoples souls as they died and do stuff like necromancy, but that's probably not possible with a single stone. A magic-user would find a lot of uses for it. You could also use it to counter more subtle types of magic like astral projection.

In the right hands it could be a terrible power for a master manipulator or a charismatic leader.