It is a sort of being that invades the consciousness of the target. It invades and completely takes over one hemisphere of the brain and completely controls it, but leaves it in contact with the other half.
Most of the time, it ignores the other half, but sometimes it works the control both sides. It results in a very apparent split personality. If the SCP takes the left hemisphere, it is able to talk. If it takes the right hemisphere, it cannot, but is still present. It is random which side it takes.
Ends up in a very apparent one half good one half evil sort of thing. Think split brain patient, just one of the sides is sinister and in constant war with the other side.

Item #: SCP-XXXX

Object Class: Euclid

Special Containment Procedures: