In the Geometric Greek period (8th Century BC), there was an interesting development from Ancestor Worship to the worship of Heroes. This was inspired by the rise of Homer and other poets.
The belief was that the Heroes had been so significant in life that they could act as go-betweens between the living and the dead.
Despite their often bloodthirsty acts in life, they became defenders, healers and oracles and guaranteed fertility of the land. The idea that they had created an access of power by their heroic acts is similar to the idea of Christian saints which came much later.
In this case, they filled a role often associated with stone circle’s as the guardians of the land. Often they were fixed to a particular region where it was believed they were buried. This was not always the case as some heroes (particularly Hercules) were worshipped all over the Greek states. Like Catholic Churches different towns claimed they had a particular hero. There was also not much of a cult of relics associated with the heroes.
Worship of Heroes was less personal than that of ancestor rituals. It was something carried out for the good of the Polis or the region. Most of the time worship was centred on old Mycenaean tombs, which were considered the graves of the heroes. However later heroes could be anyone from founders of cities, top politicians, sporting greats and top poets. Basically anyone who other people were prepared to worship after they had died.
Ignoring the hero was considered bad as they could cause crops to fail. Food dropped on the floor was assumed to have been taken by the hero and could not be picked up.
There were cases of anti-heroes. These were spirits of people who were outright nasty and continued to haunt a particular area. Often these were worshipped whatever their crime because of their ability to do harm and to go into the underworld.
This also made the words of Homer, Hesiod and other the epic poets, guides to using these spirits. We can see in the Greek Magical Paprii the words of Homer being used on magical spells presumably to link the talisman or spell to the spirit of the Hero.
Worship continued until Christianity replaced paganism, although curiously the texts of the heroic poets, which were effectively magical texts for this form of spirit work were mostly preserved.
Indeed, the idea of Saints were just Christian heroes replacing pagan ones.
The modern idea of a hero has fallen away from this. If a famous person dies we do not believe that they will curse or bless us. In fact ,it is an interesting point that dead modern heroes have the shelf life of fresh milk and are soon forgotten.
It is just as well. Who the hell would worship Kane West, Steve Jobs or any of the other shallow celebs we have created. But Military and other heroes names are also forgotten, which means that we might be missing the chance to have an underworld communicator.
Other than that there are long lists of Greek and Roman Heroes who could act as underworld go-betweens.