If you didn't know, over the past weekend Dungeons & Dragons had an online convention of sorts complete with some panels and live games. Two of the panels I found particularly cool. First was this one, with a bunch of real life bards talking about what it means to be a bard that just gives a lot of insight in ways to play a bard and why being a bard can be such a powerful identity for some people. Second was this one, with the fine folks from Asians Represent talking about how to include Asian stories in your game without stepping on harmful stereotypes.
In the second podcast, one of the panelists mentions this blog post from Throne of Salt. The article talks about how D&D does not know what is a 'monster' and what makes them. It is, essentially, an attack on the trope idea common in D&D (and RPGs like D&D) that a race is evil because of some inborn malice or evil. He also posits that a monster is made, not born. There is a story to every monster. There is a story to every conflict. And it can and should be so much more than just "this entire group/thing is evil because it is evil."
What I love about this post - beyond my agreement that a 'real' monster is made by something else - is that it also sets up an easy structure to make an adventure that can be one session or multiple sessions depending on your need. The best part about this structure is it can be truly open ended - not locking your players into any particular answer giving them, and you, maximum flexibility.
Step 1: What Is The Apparent Problem?
The apparent problem is what brings the adventurers/players to the area. This is the Champawat Tiger that the Throne of Salt article talks about. A monster, beast, creature, or thing that is causing problems and killing people. It could be a beast like the Champawat Tiger. It could be a group of pirates. It could be an Orc raiding party. It could be an army. It is the obvious threat to the area that also acts as the call to arms, call for adventure, or call for heroes.
Step 2: What Caused/Brought About The Problem?
This is where we get to be creative. The Champawat Tiger was caused by a failed trophy hunt that left a tiger too hurt to kill its normal game, and that desperation and starvation made it prey on people. This is a common story for Man Eaters, and can also work in D&D games. Now step 2 is also frequently where a lot of trope adventures drop the ball by over relying on tropes and the simplicity of "they're evil because they're evil."
For example, an orc horde could be raiding and pillaging an area of towns because they're evil and rather than live their own lives of farming and security would rather raid, rape, and pillage. However, when you do that you don't really have a story. You just have a conflict. At best you have a prolonged combat encounter. The orcs are going to fight, because orcs like to fight based on their genetics. They're going to raid, rape, and pillage because they're genetically (or divinely per 5e I suppose) predisposed towards that. There is no diplomatic solution here. At best you point them at another village or town. The only real solution is to kill them all which is combat.
However, if you try for a bit more to that story you open things up. What if the orcs are raiding because something is killing their crops and herds leaving them starving, and tense relations with the local towns means their attempts to purchase or trade were met with fear and resistance - or even just a simple misunderstanding caused the problem. Now we have solutions...which brings us to:
Step 3: How Do We Solve The Problem?
The fun part about this is you don't need to answer step 3. This is what the players do. Finding the answer for Step 3 is the adventure. How do you solve the problem of Man Eating tigers being created by trophy hunters who are taking shots that wound but don't kill? How do you solve the problem of hungry orcs raiding and pillaging for food?
I would recommend sketching out a couple of ideas: kill all the orcs (the classic answer I suppose), negotiate a truce and trade agreement between the orcs and the villagers, solve what is killing the orcs food supply, some other random crazy PC solution.
Having a few ideas in mind gives you ways to suggest things or to emphasize. Not having any answer locked in keeps you flexible and open to your PCs' propositions for potential solutions.
A Note On the Supernatural
While I will stand by Throne of Salt that monsters are made, and people/things are not evil by some internal machination, I do tend to make an exception for certain supernatural things. Demons, Devils, Angels, the Fae, and other supernatural beings are often the embodiment of these concepts as they are spun from that creation in some sense. They do not exercise free will or freedom to choose in the same way that people do. Or, if they do, they have interests and plans that are not necessarily comprehensible to mortals on the prime material plane.
By which I mean that the cause of making the monster can be the actions of an angel, a devil, or a demon instead of a person or some other hapless being doing wrong without realizing it. This still provides solutions (sending the supernatural thing away, killing it, making a deal, etc.)
A Note on Evil
Defining alignments is a great way to start a fight. So in the interest of defining my terms, I suppose I should say that in D&D terms I define Good vs. Evil as Altruism vs. Selfishness. I agree with the statement that evil as a lack of empathy. An evil thing does not care for others, they have no empathy for the plight their actions cause, which in turn allows them to justify horrendous and tragic things as necessary. Good on the other hand, is therefore the presence of empathy and consideration for how actions will impact other people. Which is where "Lawful" vs. "Chaotic" comes in, with Lawful being concerned for the whole, and Chaotic being more concerned for the individual even if it spites the whole.
A Lawful Good character may be willing to inflict suffering on a small group to save a larger group. A Chaotic Good person would consider the act of willingly inflicting that suffering on a small group as evil in and of itself. Obviously there are exceptions and wiggle room for everyone based on stakes and personal views on things. But it makes as good a way to view it as others, and is part of my view on these things. Your views may vary, and that is ok.