Although there have many books which have included a shaman class in Dungeons and Dragons, and other table top RPGs they all miss one key aspect of shamanism.
Shamanism isn't so much a class as it is a calling, for anyone could become a shaman including the small children of the Alps who Perchta led to battle, or the blacksmiths of the Celts, the blind girls of Northern Japan, and even Robin Hood. The spirits would call on shamans to help them work with the mortal world. Sometimes the local Fairy Queen might call a shaman to help her provide the people of a land with healing, or to rob from the rich to give to the poor. Other spirits would call shamans to help battle the spirits of the underworld which sought to hurt humanity, and sometimes darker spirits would force a person to help them spread illness and evil.
What's important for your RPG is that perhaps you shouldn't treat shamanism as a class, so much as you should treat it as a way of providing your characters with quests. A shaman could be a cleric, a druid, a wizard, a thief, a fighter, etc. All of these might have skills that are useful to the spirits.
There were three primary ways which these shamans would work with the spirits, fairies, and gods.
Many shamans, especially those of Greece and Japan would become possessed by the spirits they worked for. These spirits could then work through them, casing healing spells, providing advice, foretelling the future, etc. There were people who lived in caves in Ancient Greece who would be possessed by nymphs in order to help the nymphs town. Nymphs in Greek lore were often the founder and guardians of cities. More than this they were often the great, great, grandmother of the people in the city. So a nymph would choose a person to possess so they could aid their down. This person gained a companion who had a lot of knowledge and some healing spells, but little else. So in terms of a game a group of adventurers might by chosen by a nymph to protect a town, in return she can provide them with knowledge and information, some divination and healing spells (sort of like if they had a few scrolls and potions) but in truth they would gain very little but a lot of jobs to help protect a city.
In Japan there were villages of Miko (shamanistic figures) who would travel the countryside (often with a body guard). These Miko were in constant danger from bandits, from lecherous samurai, from the spirits of the dead, etc. Often they would be hired by a village to become possessed by the spirits of the dead to gain instructions on how to improve harvests, etc. The Miko might have to battle evil sorcerers who controlled armies of magical shape shifting foxes, travel into the mountains to battle and ultimately calm angry kami and spirits of the dead, or seek out stolen treasures.
The point is that while being possessed by spirits might provide an adventurer with quests it would provide only limited support in battles.
RPG's tend to treat familiar spirits as servants, as a benefit of a specific class, but in truth familiar spirits were very often the lords of the witches who served them, or companions, only the most powerful shamans had familiars who they controlled.
In some cases a familiar spirit was scent by the Fairy Queen, the Sky Gods, the Lord of the Underworld, etc. In order to direct a shaman. In such cases it might be best to treat the familiar spirit as an employer or a companion rather than an only occasionally thought of servant. Indeed you could have a game where one of the characters plays a familiar spirits who serves the Queen of the Fairies and which levels up just like the players do.
Sometimes shamans would serve evil spirits, not always by choice. There are many cases where an evil spirit would kidnap and torture a person until they agreed to serve. Often these people would curse the bride and groom at weddings, steal milk, blight crops, cause illness, spread discord, and evil kill people for their spirit masters. Such people could be anyone the spirits could torture into submission and in lore were often simple farmers, shepherds, etc.
Enter the Spirit World
Perhaps the best known shaman power was their ability to enter the spirit world. This ability, however, provides very little advantage in most circumstances. Granted a shaman can heal the sick by entering the underworld and battling or negotiating with the spirits which caused the illness. They can help the crops grow by battling to steal the fertility of the land from other spirits and shamans. However, both these things are more like quests to achieve a certain goal, rather than a power in and of itself. Often (though not always) the shaman isn't even entering the spirit world on their own, they are doing so with the help of the spirits. So it would be easy to simply say that a group of characters serves the spirits and so the spirits will bring them into the spirit world to complete certain quests.
Again the most important thing to understand about shamanism is that while there might be specific classes surrounding it, most of the shaman's "abilities" have nothing to do with a specific class, rather the shaman is a calling, an opportunity to give the players of your rpg or the characters in your story interesting quests.
Stay tuned for a few more of these quest ideas in future articles.