There are many different ways to become god of the dead. You can win the job by chance (Hades/ Pluto), you can be cast into the underworld by other gods (Hel), marry into the job (Nergal), or you can be the first person to die.
Donn was one of the invaders known as the Milesians, after their father Mil. He was the warlike one, while his brother Armaigen was the poet/judge. They eventually did take Ireland, but not easily, and Donn never got to enjoy their victory.
There are many stories in which a heroic character penetrates the otherworld and is challenged by some sort of ogre or other strange being. This challenger may question the hero’s worthiness or ability.
Norse myth has two examples of a female quester who faces a challenge to her fitness and an attempt to thwart her in her aims. In Hyndluljod Freyja has to convince the far-from-agreeable Hyndla to help her protegé win a lawsuit, while in the poem Helreid Brynhildar Brynhild is challenged on her way to Hel by a giantess who questions her ethics and actions.
Although most of us think of Pluto as the Roman equivalent of the Greek god Hades, it is one of the Greek god’s titles, usually given as Pluton, Wealthy. This referred both to the earth’s fertility and the mineral riches that could be mined from it.
Thou shalt find to the left of the House of Hades a spring,
And by the side thereof standing a white cypress.
To this spring approach not near.
But thou shalt find another, from the Lake of Memory
Cold water flowing forth, and there are guardians before it.
Say, ‘I am a child of Earth and starry Heaven;
But my race is of Heaven (alone). This ye know yourselves.
But I am parched with thirst and I perish. Give me quickly
The cold water flowing forth from the Lake of Memory.’
And of themselves they will give thee to drink of the holy spring-
And thereafter among the other heroes thou shalt have lordship.
Julius Caesar wrote that the Gauls believed they were descended from Dis Pater. In his writings, he did not give the local names for his deities, substituting ones his readers would recognize. (This was the interpretatio romana, giving foreign deities Roman names and attributes.)
Dis was originally the Roman god of wealth, fertile soil, and underground riches, who became equated to Pluto, Orcus, and Soranus. The question is, what Gaulish god reminded the Divine Julius of Dis?
Apollo seems to have made a habit of swallowing up other gods. He took over (or was given, according to later mythology) the oracle of Delphi, which had belonged to his grandmother, Phoebe. He seems to have taken over the healing role of a very early Greek god, Paean, and also an Italian god named Soranus.
We don’t know a lot about Soranus, but he was worshipped at Mt. Soracte in Etruria, an area sacred to underworld gods like Dis Pater. Like most of the Italian gods, he had a partner, Feronia, whose sanctuary stood next to his. Although his cult may have involved the otherworld and the dead, his name is probably connected to that of the Etruscan god Suri, a god of purification and prophecy.