Undead Body is a defensive ability used by Skull.
It gives the user immense durability and defensive capabilities. The attacks still cause the same amount of pain to the user, leaving it up to their pain tolerance to deal with it. Its defense is powerful enough to hold off four Varia members' strongest attacks without any lasting issues after the battle.
Even in his Arcobaleno form, Skull is able to survive having his brain destroyed by Bermuda von Veckenschtein, though he is left hospitalized afterward.