There are a lot of systems for randomly generating the map and rooms in a dungeon, but not many that really generate ideas to tie them together into a real dungeon.
I think this is mostly because it's hard, there's no such thing as a generator for original ideas. However, I like this widget I whipped up for a few reasons:
-I've found that, when running a sandbox, it's helpful to be able to take a basic hex-description-level idea of a dungeon, like "This ruin has been transformed into a testing ground which an anti-paladin uses to recruit lieutenants for his army. However, those who cleverly circumvent the tests are secretly recruited by a halfling spy working for another faction entirely" and graft that onto a sketchy or random dungeon, like, say, the dungeon Tony Dowler drew here and the rest writes itself-- this must be the arena and this must be a horsemanship test and this...
-Character generation systems which are just slow enough and just random enough that you get the feeling of coming to know the character as you roll, going "Ok, he's a barbarian and an aristocrat, how's that work…ok, he's also an orc…ok maybe he belongs to a sort of pseudo-Mongolian steppe-nobility…" are fun, so I figure doing a dungeon that way is fun, too.
-…and most systems I've seen to do it before haven't worked for me--they get caught up in details that make it harder rather than easier to integrate the material together into an organic whole. Central Casting: Dungeons is terrible, f'rinstance. You find out what kind of stone the chapel is made of after a few minutes rolling when what you need to know is why this fucking dungeon won't play just like the last one.
-I also think it just helps to have a typology of the things that matter in play, so a GM has all the options laid out as s/he begins to throw together ideas.
-Plus, in a sandbox, you need lots of dungeons. So you can roll up 5 or 10, grab 5 or 10 maps, and have some blanks to fill in at your leisure over the coming weeks.
So yeah, this generator does that. A surprising amount of the heavy-lifting is actually done by the villain generator link at the end.
Roll or pick for each category...
3. A few sessions
Essentially this is the main answer to the question "Why is it dangerous?"
1. Sadistic Architect
Some dick made this place just to watch people die in bizarre ways.
(Jokes about how every dungeon is this because DMs are sadistic are dumb don't make them.)
For example: Tomb of Horrors, Grinding Gear, that movie Cube
2. Meritocratic Architect
Someone made it to test people. Whoever survives or does best is rewarded.
It's really hard to make a good one of these, by the way, so be careful.
For example: Danger room in X Men, that wizard cave complex under Vane in Lunar: Silver Star Story
3. Fuck You That's Why
This dungeon is a funhouse in not only form but in concept: things are just there and there's either no reason (mythic underworld) or a reason so thin it could explain anything (like all the "reality damage" in Red & Pleasant Land or the dream logic in the books that inspired it).
For example: Stonehell, any randomly generated dungeon that isn't "smoothed out" afterward
4. Active Institution
This place is a business, guild, temple, etc working pretty much how it's supposed to and it's dangerous because they don't want adventurers in here mucking shit up or stealing their stuff. You can roll up an institution here if you like:
Institutions, roll d20
1 Alchemist's lab
6 Assassin's den
8 Guild hall
9 Spymaster's headquarter's
11 Livestock dealer/breeder
12 Market hall
13 Nest of criminals
For example: Library of Zorlac, Dark Tower, Zoo of Ping Feng, the thieves guild building in that Lankhmar story with the spider in it
The simplest kind of dungeon--it's here because someone lives here. Almost identical to an active institution above except not as complicated--it doesn't have to have any particular product or service it manufactures for internal use or for the outside world.
For example: Caves of Chaos and The Keep from Keep on the Borderlands, Steading of the Hill Giant Chief, House of the Medusa in Vornheim
While a small castle might be basically a lair/home, a true palace with extensive facilities or trade goods or administrative functions can be treated as both an active institution and a lair/home.
6. Caged Threat
This place's main function now is to keep the threat in rather than keep you out. In practice, keeping you out generally helps keep the threat in because you might inadvertently release the threat by looking for loot. A lot of tombs in games are basically this.
For example: Death Frost Doom, any prison.
The main purpose of this place is to protect the valuables inside. Nobody really lives here. (Thanks to Gus below for reminding me.) If a tomb has no undead in trapped inside, it might be this.
For example: That place at the beginning of Raiders of the Lost Ark (note the spiders and bugs in there aren't actually the threat--just gross. If they were, it might count as…)
8. Abandoned Then Infested Place
The most common kind of large dungeon, this place started life as something, fell into ruin, then beasties (often more than one kind) crept in and took up residence.
For example: Caverns of Thracia, Dwimmermount, Mines of Moria, most megadungeons and ruined cities
9. Active Then Infested Place
This place wasn't dangerous until a second ago--but now it's a dungeon because things in it are trying to kill you. Unlike the active institution, it's possible nobody would mind you being here on a normal day.
For example: The Nostromo in Alien once the alien shows up, the area Forgive Us takes place in, most buildings in zombie movies
10. Not A Dungeon To Them
Nobody wants this place to be dangerous to you or even particularly wants to eat you, but this place is simply dangerous by dint of its natural function. The inhabitants may be too mindless or alien to realize they pose a threat to you.
For example: House of a giant so big he doesn't even know you're there, the inside of the patient's body in Fantastic Voyage
11. Roll d10 on this table twice, re-roll one if you get a duplicated result
Example of two rolls: Undermountain (Fuck you that's why, Sadistic architect), Castle Amber (Fuck you that's why, Lair/Home)
12. Roll d10 three times, if you get a duplicated result, roll twice more.
Example of three rolls: the two larger dungeons in Red & Pleasant Land (Fuck you that's why, Active institution, Lair/Home)
Example of four rolls: Ruins of Greyhawk (Sadistic architect, Pedagogical architect, Abandoned then infested, Caged threat)
Roll d8 if you like life complicated, roll d20 if you don't
The status quo has just been interrupted. This is kind of like a layer of "active then infested" above except this change may make the dungeon more accessible rather than less. For example: an earthquake opens an entrance into a buried pyramid-- but it might also render walkaways and ceilings unstable, or goblins may have recently invaded from the hills, battling the lizardman inhabitants in the halls.
There's an over-arching "thing" or trick to the dungeon, some magic complexity that enforces a weird logic on events, structures or creatures inside. Like: all the rooms are spheres nested one in the next, or moving objects in one room alters physical laws in all the other rooms, or the monsters must be killed in a specific order or they auto-resurrect. The whole dungeon is, in a sense, a big trick room.
D100 ideas here.
3. No creatures
There's no monsters, only traps, puzzles and the like.
4. Universal rule
This is a simpler version of meta-weirdness--there's just one simple unusual thing. Divine magic doesn't work or it's too hot to wear metal armor or you can't hear anything or you can hear everything and every noise in the entire complex is audible in every room.
The dungeon is itself mobile, or some of the rooms are. Why is on you.
6. Time constraint
If the PCs don't do something in time, some terrible change in the situation occurs. Players can be informed of this by an NPC, a visible timing mechanism, or in some other way.
7. Staged access
There are some rooms or areas that aren't accessible without finding some secret door, key, or like item that's elsewhere in the dungeon. The main thing for the GM to remember when designing the place is is: the players are probably going to have to go back through rooms they've already been through in order to search for the thingy.
8. Doubly unusual
Roll two results on a d8.
Hey dungeons are tough enough as it is, right? Why complicate things?
Roll d8, or d6 for low-magic/low-weirdness areas
2. Compound (multiple buildings)
3. Typical large building for the area and function
4. Ruin or caves
5. Partially aboveground (roll d4), partially below
6. Traditional dungeon (below ground)
7. Magically disguised as an ordinary structure
8. Weird (floating, alive, magic hedgemaze etc.)
This is the number of mastermind-type creatures in the dungeon. (If there are no creatures in the dungeon, this is about the architect or architects). Roll according to the size of the dungeon.
A few sessions
Roll villains here.
You can get a map here and stock it using these rooms or you can use a Madlibs dungeon like this or this or this.