Immortals is a broad term for many of the entities of Greek Mythology. Immortals are people who can't die, but instead are sent to Tartarus when "dead". They never really disappear, they simply take a few decades or centuries to get out of Tarturus. Ichor is their blood, taking on the color of gold in every myth. It is said that immortals are invulnerable to any mortal steel, such as iron, copper or gold. They only are hurt by weapons wielded by gods or humungous strength such as when Hercules broke off one of the horns of the river God Achelous.