Mille Kilos

A bulky gray skinned undead wearing prisoner's clothing and carrying around on his back a very heavy metal boulder that's attached to his metal harness by heavy chains. He has a keyhole on the chest of his harness and on his metallic boots.
They were once a notorious criminals who committed heinous crimes and were sentenced to life imprisonment in International prison. Their crimes weighed heavily on their conscience, and the weight of their sins manifested physically as a heavy metal boulder chained to their back.