Paxton Page was a scientist who created the cobalt bomb. Because he refused to sell his creation to anyone, even the US government, he was branded a traitor. He continued his research as a recluse and was driven mad by the cobalt radiation, which also gave him super-powers. He took on the identity of Death's Head and drove off any unwelcome visitors to his estate.
When his daughter Karen returned home, to try and get away from her grief over Matt Murdock's death, Daredevil followed her, in order to explain how he had killed off his alternate identity of Matt Murdock. Death's Head and Daredevil fought in Paxton's laboratory, during which a vat of molten cobalt was about to spill over Karen. Paxton pushed her to safety and was killed by the liquid.
Powers and Abilities
Death's Head could manipulate cobalt radiation, which he could project as fireballs at others. He could also cover weapons with this energy, and could irradiate and mutate other creatures with it, like his horse.
Page was a skilled physicist, specializing in the study of cobalt radiation.
Page's strength was increased to superhuman levels by the radiation that caused his powers.
Page was driven insane when he gained his powers.
Death's Head carried a sword and a bola that he irradiated using his powers.
Death's Head rode a horse that he had mutated with cobalt radiation - until the apparent death of the steed.