The boy lay on the bed, his body thrashing from side to side, between the screams he chanted in a primal language, the sounds of barking dogs and howling winds, he screamed at the skies. As foam began to rise from his mouth, his body began to twist like he would break, his skin became wind burned and pained, and then stopped. He rose from his bed and stepped lightly on the floor. The wounds to his body still showing, but the lightness of hit step not showing them at all. Stepping out of the hovel he approached the tax collector and touched him gently. The blast of air sent him clear of the village. The wind howled a few more minutes and then the boy stopped in his tracks and looked around.
The woman running from the nearby home screamed in rage and fear, "NO! YOU DID NOT HAVE TO! YOU CANNOT HAVE MY BOY". The screams of the woman distracted the boy for a moment, and he turned his head, "I only have need of him for a moment" and then flung himself into the trees. The skies began to grow dark, the trees began to whip with wind and fury, and the funnel cloud built over the skies, the boy's body fell to the ground broken and bleeding, and the village prepared for a worst disaster than they could ever imagine.
The spirits are always willing to work with people, they are not willing to work with civilization. Most of those who work with the spirits are the witches and hermits who live alone outside the places of humanity. They give tribute, listen to the spirits, and keep out the further press of humanity destroying the trees. Sometimes the elves will take in these strange humans and tend to them, but witches and hermits rarely appreciate this community, desiring to be primarily in communion with nature.
The nameless spirits of nature always have need of people like these witches. Spirits are very limited to their domain, they are only able to obey the ancient laws set down for them, and behave in the ways they have since the dawn of time. Every river remembers when it was the sea - but it will never stop being a flow of water unless it is moved by a being of will. The spirits have great power, the ability to split rock, destroy a city with a wave, or regrow a forest, but they are limited to these laws. But this relationship between the hermits of the woods and the spirits is the key to this difference.
The spirits can ride a person. This means the person takes the power of the spirit into themselves, and give the spirit a shard of their own will. Spirits and People do not properly merge in a way that works, taking a spirit into their own body is horrific, ripping the person's body, and forging a compact which often kills the spirit ridden, but always ends with a massive change to the natural or civilized world.
In order to be ridden by a spirit, the first thing that needs to happen is a duel of wits - both sides must create an agreement as to the use of the body, since the power must be able to be contained within the body. The spirit must limit its powers to certain uses and purposes. The Body of Argument skill should be one of: Folklore, Bargaining, Rule of Law, Doctrine, or, Religious Diatribe, the spirit should always be using its Strength. The skills from the Body of Argument can be used as part of any Dueling Action, assuming the player can explain how they are using their skills. Whenever the human involved loses Body of Argument, they can choose to take a wound equal to the loss of BoA instead. So being thrashed by a Strength 7 River Spirit and losing 5 Body of Argument can cause a B5 wound to the Witch instead as she allows the spirit to change her body to fit the spirit, instead of attempting to make the spirit inhabit her functional body for more time.
When the duel is over, both sides have agreed that the body will be shared. The human getting the body first, and then the spirit fulling controlling the body for an equal amount of time. If the spirit wins the duel, it has decided what things the human is allowed to do with its power, and the human can use compromises to reduce the amount of time the spirit gets to use the body, or to request additional powers or scopes they can use their power within. If the human wins, they are unlimited in their use of power, but the spirit can use their compromises to limit to increase the time they get the body, or limit the scope of powers.
For example, Tirian is a boy who has spoken to the witch Tigath at the edge of his village. Having heard stories and listened to the truth, he had decided to attempt to save his family farm by harnessing the north wind. He enters into an agreement with the North Wind to be able to control the winds, and be allowed to blast individual people. Tirian decides to take a B6, B4, and B4 wound - leaving his body almost completely broken (two lights and a Midi) when he no longer is being ridden - but he gains control of the body. The spirit decides to use its Minor compromise to allow itself to be unlimited in what it can destroy. Tirian is completely without limits in his power, which was perhaps more than he wished to give the spirit, but the small limits he was hoping for left him open. If Tirian had been wiser he might have said "I need the power to drive the tax collector from my town.", limiting both the people (agents of authority) and the scale of power (he does not need to be able to level a town to do that), but Tirian was rash.
While being ridden by a spirit, the body is unaffected by wound penalties, though will often look broken and battered. They have the ability to command the elements, the spirit can simply roll its Strength and use the Natural Effect scale to figure out the scale of its effect. Sensible witches use the initial bargain to limit the maximum size of the effect, as well as potentially targets. The type of spirit bargained with will define what type of elements can be controlled - but none of the laws of the nameless spirits laid out in spirit binding apply, since the Will of a Mortal is brought to bear. The spirit then gets reign over the body, allowed to do whatever it wishes, though it can only use its powers within the limits in the agreement. The spirit can use the physical attributes and skills of the body, but it uses its Strength in place of the Will and Perception of the body. This is how spirits will enter buildings and make demands, or, in the worst case, do horrible things like burning down a building or striking someone directly with lightning. Use the effects from Spirit Binding if you need direct inspirations on what elemental control can do.
A Burning Wheel Monster Every Week