"Maxim 1: Pillage, then burn."
- – The Seventy Maxims of Maximally Effective Mercenaries
Adventurer is a title given to individuals whose occupation is, obviously, adventuring. "Adventuring" implies going on adventures, but in practice is more likely to involve murder, security detail, object and artifact retrieval, tomb desecration, and waking things best left sleeping. Adventurers usually have short lifespans, but make up for it by wielding phenomenal cosmic power and carrying around enough material wealth to destabilize the economies of entire regions. Usually Adventurers are also Player Characters. The inverse is even more common.
Broadly speaking, an adventurer's job description will fit into two categories, with some overlap. Both involve killing things, but one is slightly more social than the other.
- You belong among the murderhobos
- You belong in a pile of corpses
- Run away from any roleplaying
- You belong in a hack and slash campaign
Murderhobos is a term used (originally pejoratively, but occasionally affectionately) for the player characters in RPGs, both in video games and tabletop games. The term arises due to the fact that most adventuring characters and parties are technically homeless vagrants, generally living on the road and sometimes in temporary accommodation, and the default solution to problems faced by the typical adventurer boils down to killing things until the problem is solved or treasure is acquired. In many games (especially older pure hack and slash-types of the type that Gary Gygax despised) killing things and taking their stuff is simply the order of the day, all morally acceptable and proper, either because that's all the players are interested in doing or all the GM can come up with. In more nuanced settings, "Murderhobo(s)" is used especially to refer to characters (or entire parties) of looser morals who tend to regard massive collateral damage as an inevitable and unremarkable consequence of their actions, or who are quite happy to slaughter otherwise friendly NPCs at slight provocation or the prospect of financial gain (thus often overlapping with munchkins).
Although it is a fantasy standard that adventuring parties are welcomed into towns and villages and hailed as saviors (or at least obscenely rich individuals that are loose with money), there is a small trend for this to be inverted in some games and stories (usually for comedy's sake) and have the protagonists be treated as the homeless serial killers they actually are, either by having them rejected from civilized society or by having the NPCs/minor characters respond with pants-shitting fear whenever the heroes present themselves. Murderhobos left to their own devices are bad news for the region they occupy, so they are often dealt with by giving them quests that take them to dangerous places in distant locations, where they can kill some other monsters (or at least some foreigners).
See, in the dim and dismal early days of RPGs, especially Dungeons & Dragons, Roleplaying Games did not put much stock into, well, roleplaying. Gameplay was seen less as a form of collaborative story telling and more as a conflict between the DM and the players, something to be won. Remember old school D&D and it even older predecessor Chainmail spun out of table top wargames where there was a definitive loser and winner. Nevermind that such a conflict is inherently tilted in the DM's favor, since he can always just drop rocks on all the players at any time he feels like, making a "victory" by the DM not actually any accomplishment. This mindset was encouraged with competitive tournaments held at conventions to see who could complete a module in the shortest time and with the most gold and Experience Points. Most campaigns were just a series of dungeons crawls leading up to the final battle with the dragon on the twentieth floor, where the only NPCs were the enemies meant to be killed.
Even when the games moved out of the dungeons and into the wider world, some DMs kept this overly competitive mindset alive. The only difference between outside and inside was the size - everything and everyone was still part of Team "Destroy the PCs" and out for blood. That captured princess you just rescued? She's actually a shapeshifting monster and eats you while your guard is down. Those merchants you brought gear and food from? Swindlers whose stock is non-functional at best, and at worst sabotaged so it'll break at the worst possible time. That otherwise apathetic level 1 Commoner you just passed by the street? They'll happily rat you out to the BBEG's minions for a couple copper coins because you dared breathe in their presence.
Having an actual backstory made this even worse, as it amounted to generously providing the DM a list of people who would instantly recognize you, know all your weaknesses and abuse both without hesitation. The rare few friendly NPCs who weren't faking it were just ways to attack the PCs indirectly: Got a little sibling who idolises you? Boom - they've been kidnapped by Ogre bandits; better save them before they get raped! Got a family or a sweetheart back home that you're adventuring to raise money for? Boom - they just got unceremoniously murdered off-screen by the BBEG's assassins to provide a cheap plot hook for an angsty revenge story! That Lamia child you rescued from an untenable situation to raise as your own? Boom - she's always been planning to slit your throat in your sleep, and now you have to kill your adopted daughter!
Hence, the development of the murderhobo: a character functionally hatched from an adventurer egg who shoots first, asks questions never, and doesn't give a damn about anything that doesn't fuel their progression of loot and EXP.
Murderhoboism is invariably a metagame effect. There are two general reasons as to why. Firstly, it can be a conscious decision by the player to not have attachments that a bastard DM could exploit to force a dramatic action. Secondly, it can be a response of a player for whom the only interesting part of D&D is the combat. Role playing and fluff? Nah, they love crunch, mechanics and big numbers, they made a character with enough DPR to one shot a lich and by GOD they're going to use it! These two different responses to gameplay need two slightly different responses from the DM. To head the first one off, the DM needs to make it clear to the players that such things won't happen (unless you're rolling psykers in Dark Heresy). It's up to you, Mr. DM, to design the story so the players have ample opportunity to stop harm from befalling their loved ones, and in turn you can get NPCs that act as quest givers and sources of aid and support.
The second type of player is more difficult to deal with, as they fundamentally have a different form of engagement to your game than other players. That's not inherently a bad thing and countless groups have that one player who tends to be silent when there's not a puzzle or combat to be had. The problem is when they act out and let it harm the enjoyment of other players by murderhoboing when other players are engaged with the story and narrative. In this case you should talk to the player on the issue, but at the same time if they keep acting out: don't be shy to drop kick them out of the group for everyone else's enjoyment. In general, fighting murderhoboism is win-win for everyone, even players with slight murderhobo tendencies.
Alternately you could lean in to it. A hypothetical party of evil adventurers would probably default to some degree of murderhoboism and mercenary work without a longer term goal to direct their purpose toward, and to be fair some times being a cackling Skeletor villain with zero self awareness can be funny. Even so, too much murderhoboism could also cause problems if the reaction to the murderhoboing causes the players to stop having fun. Just be aware of the kind of game you're trying to make and tailor the world, both in game and on a meta level, to what you expect will bring the most engagement to the players.
Mercenaries are people who fight in conflicts for the sake of direct profit provided by their employer. In most fiction, they exist to provide a group of people who have military force capacity, while not being associated with particular political and societal factions. This means that most mercenaries are used for protagonists and anti-heroes, as it's much easier to create a believable and unique character without restricting them to a set group of beliefs of a faction. Equally frequently, the mercenaries are shown in the dark light as well, being pragmatic villains who have little interest in anything besides money, thus allowing them to be the bad guys by doing anything and everything evil for the sake of wealth.
The difference mercenaries have though is that they actually existed. However, real mercenaries tended to be amoral (often Swiss) pikemen who would serve as royal guards for monarchs too paranoid to trust their own people to not murder them. The ones that DID actually go and fight wars, they tended to be Germans and were often as not in it for the looting. And if the other side offered them a better deal, they were almost always more than happy to betray their now-former employer. Some, such as Francesco Sforza, even ended up as prominent aristocrats themselves. Needless to say, the aforementioned unreliability is one of the reasons why modern nation-states use their own standing armies to fight wars now (with some exceptions, of course).
In many a game, the PCs are hired as mercenaries to provide services for someone, starting from removal of Giant Space Hamsters from the cellar of a local noble, to poisoning the king during a banquet so his brother's cousin's sister can usurp the throne. And, since parties can range from Chaotic Stupid to Stupid Evil to Stupid Good (even at the same time), all characters can act towards a common goal without having to resort to fratricide because they can't agree on an approach to the situation (as most players use their status as mercenaries as an excuse to do things out of character, because "Derp, Money").
Adventuring as a Job
Recently we've seen an influx of new settings where being an Adventurer is an actual, real job description. Often employed in an Adventurer's Guild, this trope plays everything about the typical PC completely straight, but builds it into the setting as an accepted and regular part of life. Need to fix a house? Get a carpenter! Need a troll dead before he steals all your sheep? Get an adventurer!
Increasingly common in medieval animu, it does see some use in western settings as well (wild west gunslingers and pirates of the sea and space variety all fit the adventurer mold well). Order of the Stick has run with this since the very beginning, but since its a self-aware pastiche of RPGs in general, that's to be expected. Many a DM create similar worlds, since that completely circumvents the need to make a group of wildly different people of different creeds and races come together for realistic reasons. You can just say that they are adventurers for hire, and Bob's your uncle.
- See here for a man who fights like a murderous hobo.