In the section on "Dungeon Master Information" in the Moldvay Basic rulebook, Tom Moldvay provides a step-by-step guide to creating a dungeon, starting with choosing a scenario:
A scenario is a background theme or idea which ties the dungeon together. A scenario will help keep the dungeon from becoming a boring repetition of "open the door, kill the monster, take the treasure."
Tom Moldvay, D&D Basic rulebook (1981)
Frontispiece to module B3 "Palace of the Silver Princess". Illustration by Bill Willingham.
1. Exploring the Unknown
The party is hired to map unknown territory. The area might have once been familiar but is now overrun or destroyed; a strange tower might mysteriously appear overnight in a familiar area. Dungeon modules B1 (In Search of the Unknown) and B3 (Palace of the Silver Princess) are examples of exploration scenarios.
2. Investigating a Chaotic Outpost
This scenario has to do with a Chaotic invasion (either in progress or about to begin). The characters must enter the enemy outpost, find out the strength and plans of the invaders, and destroy the outpost if possible. Dungeon module B2 (The Keep on the Borderlands) is an example of this type of scenario.
3. Recovering Ruins
The party is usually scouting an old village before permanent settlers move in. The ruins have often been overrun by a specific kind of monster which must be killed or driven away. The ruins could be part of (or underneath) a thriving town!
Zargon, from module B4 "The Lost City". Illustration by Jim Holloway.
4. Destroying an Ancient Evil
The evil is usually a monster or NPC (the exact type not known by the players). Sometimes the evil has been deeply buried and re-awakened by recent digging. This theme is often used along with others; for example, an ancient evil may have to be destroyed before some ruins are resettled.
5. Visiting a Lost Shrine
To remove a curse or recover a sacred item, the players must travel to a shrine which has been lost for ages. The characters usually have only a rough idea of its location. The players may have to consult an oracle or seer during their visit.
6. Fulfilling a Quest
This is a scenario in which a king (or other NPC) provides a reason for adventuring. A variation of this is a special mission for "the gods". Quite often this scenario also involves the recovery of a sacred object or powerful magic item.
7. Escaping from Enemies
The player characters begin this adventure as prisoners, and must escape. The reason is clear and simple, especially if imprisonment is to be followed by the deaths of the characters. The DM must be careful to make escape possible (though not necessarily easy).
8. Rescuing Prisoners
Valuable and important persons are being held prisoners by bandits, a tribe of orcs, or an evil magic-user. The party sets out to rescue the prisoners because they have been hired to (for an expected reward), for a debt of honor, or for some other reason. Sometimes the player characters are only hired to guard an individual who is talking over the demands of ransom. This scenario is the basis for the sample dungeon hereafter (The HAUNTED KEEP).
9. Using a Magic Portal
The "magic portal" is a device which magically sends creatures from one place to another. A magic portal is usually a door into another dimension or world, and thus may easily become the point of an invasion from one of these worlds. Portals may be known or secret. If known, the characters may be on a mission to destroy or guard a portal used by enemies, or perhaps to reopen or repair a "closed" portal. Secret portals can be used to make sure that the characters will visit an important area of a dungeon. Portals might operate both ways, or one way only (teleporting into but not out of an area).
Moldvay uses a magic portal in the Expert D&D module X2 "Castle Amber (Chateau d'Amberville)" (1981) leading to Averoigne, a province based on the writings of Clark Ashton Smith, in a parallel world similar to medieval France.
A variation on this type of scenario is used in module DA1 "Adventures in Blackmoor" (1986) in which the player characters pass through a gateway into the ancient past.
10. Finding a Lost Race
The players find a once-human race which has lived underground for so long that it has begun to change. Its members might have developed infravision, changed color, or begun to fall back into animal ways. This scenario works well when used with Destroying an Ancient Evil, since Lost Races are often servants of the ancient powers. This scenario requires extra work and imagination by the DM, since details for the Lost Race must be invented.