Source: The Outer Limits
Location: Another dimension, and nuclear research facility
Threat Assessment: 6. The energy needs a shell to exist, so it flows into people wearing suits that were suppose to be proof against radiation. They are intelligent, and don't feel pain. If you destroy a suit you don't destroy the energy.
Limitations: They can't operate far from their dimensional rift. So it's best to close that sucker off.
Personal Note: For some reason these scared me as a child.