Hades

Hades is a brother of Zeus and Poseidon. Not hard to be that he also a son of Kronos. There are several theories about how Hades got to the tartar – the underground world. According to one theory he was thrown there by his father Kronos. Kronos was afraid of possibility to be driven away by Hades and so he threw him away from Olympus to the underground. And that is how Hades became a god of underground world, the world of dead souls.
According to another point of view Hades was proclaimed as master of underground world after Kronos was driven away by Zeus. Three gods tossed and according to results of toss Hades became a god of world of souls of dead people. There was a dog near the gates of tartar. The name of that dog was Cerberus and it was a guarder of gates. Cerberus had three heads and those were the evilest heads.
Hades had a helmet that made him invisible. With that helmet he made himself invisible even for gods. Though, Hades never left his world and always remained under ground. He was also considered as god of fertility. People believed, that fertility power of earth is concentrated in its depth, and those depths were in realm of Hades.
He was considered as the most dull but at the same time the most generous god of all known. It was so, as there was no such person who wouldn't be gifted by Hades with eternal peace. Everyone was allowed to get to tartar, and sometimes there were people who were allowed to get back of the tartar.