What are adventurers? Adventurers are best understood as a form of natural disaster, like a tornado or earthquake. Their lives are whirling clouds of intrigue and violence, and normal people who value life stay as far away from these passive-aggressive psychotics as they can.
The best of them are paranoid, touchy, greedy, and bloodthirsty; they are likely to respond to nearly any challenge or obstacle with immediate and overwhelming force. Even "good" ones are killers, grave robbers, and notorious lechers. The worst of them are mass murderers, thieves, and extortionists. No matter how grim the situation you find yourself in, they will take action only for money, and will always want more than you and your village are prepared to pay.
Despite their greed, they have wealth beyond imagination- the equivalent of the wages of several lifetimes of skilled labor- all invested in armor, weapons, and other various and sundry nastiness. Sane people, upon becoming this wealthy, would buy a large estate and mansion and retire. These lunatics buy bigger weapons and go after more money- your money. They earn this money in ways which boggle the mind and churn the stomach.
They think nothing of walking into lethal situations normal and sane people shun. They impassively face armies of well-armed and-equipped humanoids, the risen dead, and demons from the pits. The most hideous and unnerving monstrosities are slain without even a hiccup, without a blink, twitch, or grunt to betray any kind of emotion. Adventurers are beyond brave, beyond fearless, beyond impassive. They are simply incapable of normal, human fear. Death has no meaning for them. It is simply another annoying obstacle.
If their own life means little, the lives of those around them mean even less. They will kill without compunction, or even a moment's thought. They treat everyone as if they were a potential enemy and treat every situation as if violence could break out at any moment.
Buy their services, if you must, but don't expect gratitude or loyalty. They will try to take whatever isn't nailed down, and try to nail anyone who catches their fancy. Don't expect grace, courtliness, manners, or for that matter even barest civility. Adventurers act like- and apparently believe- that they are the most important people in the world.
Your laws and mores matter nothing to them. Most don't even remember such concepts exist, and upon being reminded of them react with extreme annoyance. This usually equates to extreme violence.
If they are welcome, it is only because things have gone very badly and they are moderately less dangerous than whatever supernatural evil is about. Once they have done whatever it is you need, reward them with gold (of course, try getting them to act without it) and invite them to leave, immediately if not sooner. Asking them to stay is a recipe for disaster.
-RPGnetter Jasyn Jones describing how he runs D&D.
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