Powers: interdimensional travel, merge with shadows
He and others of his alien race have learned to possess humans by merging with their shadows. He has also had the technology to become a "shadow duplicate" of someone - power he used to become a duplicate of the Hulk.
Weaknesses: While he is possessing someone, if the person's shadow is diminished or eliminated, he can lose control of the person and possibly cease to be a threat. Similarly, if he is a shadow duplicate then strong lights can eliminate his shadow form, and regaining his normal form could take months.