so let me just explain to you what this is and how it affects simple, warning: he is the only error infected to this plague because simple was the first one to be injected with it and simple destroyed it soon after, therefore nobody else even had a chance of getting infected,

So error's plague is a chemical mixed with soles, once it has been mixed together it turns into this liquid-like substance and is very dangerous, basically the only way to inject it into somebody is to cover them in it which is what happened to error, once it was injected the plague went into the host and taking over the mind, and it forms a new being inside the host's body, (that is in this case unknown)

The host could go insane with it and killing himself some how or be in tons of pain while two minds try to fight for control, (error ended up surviving). But the second being is still in his mind and when he wants to take control his body would be covered in a blood-like liquid that is black, and the x eye would show when he is in control, this requires a lot of pain and is painful to take over.

Once their in control they can have immense powers like teleportation and possibly killing others using the plague. And that is an explanation of what unknown is, he is not a second version of error he is a being within error that takes his form and is trapped in there until error die's so just to clarify unknown is using errors body he doesn't have one on his own.