> But what can a spirit do? I've played clerics and wizards for many years and understand their spells and powers, but I've never played a Shaman/animist. Even though I don't play D&D anymore, the majority of my fantasy knowledge comes from that game. I understand getting information from a spirit would be useful, but I thought spirits are incorporeal, like ghosts. And if they could throw things like a poltergeist, how could you stop them if you could not harm them?
Spirits may enact a spell effect for you. Rather than casting a spell yourself, you ask or command one of your spirits to do that, or you collect a favour from a local spirit. Not too different from a theist asking the deity to grant them a magical effect.
You might want to send a spirit to intercept another spirit, or a magical effect directed at you or one of your friends/interests. So, if a hostile spirit attacks you, a friendly one might help you by weakening the hostile, by shielding you, or by granting you the power to hurt it back, however weakly.
Finally, spirits may carry their magic to a target. Unless the spirit is bound to you by magic or obligation, this may be a one-time use of that spirit, or require that you renew your ability to control this spirit (or one like it).
Spirits might aid you in other ways - warn you, detect things for you, hide you, strengthen or heal you, alter your perception, grant you skills, possess you (some might have to possess you to grant you skills or effects, or to hinder hostile spirits from affecting you).
Sending something like an angry ghost against another person, in order to possess or drain or simply hurt that person, would usually be in the realm of expert animists (shamans, possibly summoners using alternate methods of interacting with the spirit world), or in the realm of demonologist sorcerers, whether through sheer will and force or through hideous pacts.
If the animist faces people who hardly ever interact with spirits, his exotic magics might give him uncounterable powers. However, the reason people have little interaction with spirits usually is that they or their specialist magicians created conditions that weaken spirits so they avoid these people or lands.
Cultures often at odds with animists will either develop effective countermeasures, or else be overwhelmed.
Incorporeal spirits may still have weaknesses to material stuff.
The Elemental correspondence offers weaknesses for elementally tied entities - if you want to overcome an air spirit, use earth (hallowed earth, or crystals, or...), to overcome a spirit related to fire, water always is a good bet.
Cold iron is detrimental to spirits.
Magical barriers may exist for incorporeal entities that are mostly ignored by corporeal entities - a hearth or a clan tula will provide certain levels of resistance to hostile spirit activities, as will sanctified or sacred ground.
Anything that changes the nature or strength of the magical background may affect spirits, too.
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