Among the Shuar people (formally known as the Jivaro) of the Amazon rain forest, the arutam spirit (or soul*; wakani in the native language) is the acquired spirit of a dead man, which is used to protect a warrior. The arutam spirit is different from the muisak spirit, which must be aggressively captured through hunting and shrinking heads of your enemies; the arutam spirit comes to you voluntarily, and is the soul of an ancestor, not an enemy.
Among the Shuar, the spirit world is considered more real than the 'real' world, so all types of spirits are very important. There are three basic types of human spirits: the nekas spirit, which is the true (or 'ordinary') spirit; the muisak spirit, which is an avenging spirit; and the arutam spirit, which is an acquired spirit. The arutam spirits are associated with wind and thunder, and there are thousands and thousands of them. This is the type of spirit the Shuar spend the most time worrying about, as they give you protection from you enemies.
Arutam literally means vision; arutam wakani is a vision spirit. With any luck, a person's first vision comes shortly after birth; when it is a few days old, a baby is given tsentsema (a comparatively mild drug, by Shuar standards), which may help it see an arutam, giving it a dose of supernatural power which will help it's survival. Seeing an arutam is good enough for a small child, but sometime between age six and puberty, a male child had better get an arutam spirit of his own if he wants to survive to manhood.
To gain an arutam spirit for the first time (and this is also the most common method of gaining one thereafter), one goes on pilgrimage to the largest waterfall in the area (within one day's journey). Every pilgrimage will include at least two people. Once there they pace back and forth through the water, chant, fast, and drink tobacco water. If no one sees an arutam spirit, they will head home after five days. But if someone feels that they need to see an arutam spirit, they may take maikua (Datura arborea), a drug that causes excitement, delirium, hallucinations, and narcosis. Only one person will take the drug, and the other(s) will provide moral support -- and physical restraint when the delirium hits. Even if there are enough people in the party to restrain another drug-taker, the arutam spirit won't appear to a coward, and unwillingness to take the drug alone is a sign of cowardice.
The arutam spirit may appear in a number of forms; two jaguars fighting, two anacondas fighting, a large ball of fire, or a large disembodied head are the most common. When the arutam spirit comes within 20-30 feet of the viewer, he must run forward and touch it. Most arutam spirits will explode violently, although the giant fireball just disappears.
After the pilgrimage the pilgrims return home, without telling anyone if they were successful or not (if you tell someone you have the spirit, it will desert you). If a person was successful, that night the arutam spirit will appear in his dreams, and enter his body. The spirit will give the bearer protective power, called kakarma. This power will make the bearer stronger, smarter, and will prevent him from committing dishonourable acts. It also makes him immune to death by physical violence, sorcery, or poisoning; it will also increase his resistance to disease, although it cannot protect him completely. The change in personality due to these factors may be great enough to alert others to the fact that a person has gained an arutam spirit. Finally, it will also give him a tremendous desire to kill. (This is the reason most females don't acquire arutam spirits, although they may. If a female does acquire a spirit, she will probably kill through poisoning, not in battle).
Usually within a few months a killing party is mounted**; the warriors gather in a circle, and describe to each other the arutam spirits that they saw. As they do so, the spirits leave their bodies forever, although they may later enter the body of another Shuar. The power given by the spirit remains, although it will slowly ebb away over the course of the next few weeks. The group attacks; if they kill, they are free to gain another arutam spirit as soon as they whish; if they do not, they must choose another victim and try again. In theory, they are still invincible; if one dies, this in only evidence that he had lost his spirit without being aware of it.
Gaining a new spirit will stop the power given by the lost one from ebbing away; this fact can allow a person to gain a large amount of power by continuously gaining new arutam spirits and killing. It is also important to keep a steady turn-over of arutam spirits through yourself in order to prevent them from wandering off from your body. After four or five years, a spirit will start to wander while you sleep, and another Shuar may capture it as his own. This might be done accidently through a vision quest, but it might also be done intentionally, probably because the other party wants to kill you, and cannot while you are protected by an arutam spirit. This theft is accomplished by drinking natema (a weaker drug than maikua, but still serious stuff), and beating on a hollowed-out log signal drum, while chanting the name of the intended victim. This theft will give the thief another arutam spirit.
It is always beneficial to have two arutam spirits, primarily because this means that you cannot die of any cause whatsoever. Also, if someone tried to steal your spirit, he will probably have to get a friend to help him by taking the second spirit, as no person can have more than two arutam spirits at a time.
When a man dies, he spawns a brand new set of arutam spirits, one for each arutam spirit he has held during his life. Arutam spirits are immortal. (I don't know if the Shuar understand the mathematical implications of these two facts). If you have held any arutam spirits at all, you will also produce a muisak spirit, which will try to kill the murderer (or, if the murderer has an arutam spirit of his own, a close family member). Every person has a nekas spirit, which will wander off, undergo a number of transformations, and eventually turn into mist.
* There's some problem with translation here -- these are basically souls, but they are created at death; they literally don't exist until the person of whom they are the 'soul' of dies. They then wander around on their own. They aren't limited to being a 'part' of a human, but they are very much connected to the human world. (Or the human world is closely connected to them.) Anyway, they aren't exactly souls as they are thought of in the Western tradition, but you probably shouldn't think of them as some sort of supernatural beings separate from humans.
** Killing is one of the prime occupations of the Shuar; they may be attacking another group of Indians, or a nearby family of Shuar (they hold grudges for a long time, and are big on revenge, and giving revenge on those who gave revenge upon them, and etc.) Besides satisfying the arutam-created desire to kill, these raids also let you capture enemy wives and children (which you can then make your own), and bring back a muisak spirit, which can increase your kakarma power (or the power of any that you choose to share it with).
The information in this writeup comes from The Jivaro by Michael J. Harner. It's a great book about one of the more interesting (read: weirder) of human cultures.