5. A Lich
"Often such a creature is the result of a transformation, as a powerful magician or king striving for eternal life uses spells or rituals to bind his intellect to his animated corpse and thereby achieve a form of immortality."
The most important thing to take away from the lich is someone of great skill who has turned from the good fight to evil, selfish ways. Traditionally it's a wizard of the highest order who then starts looking to gain power beyond compare - often through dark, forbidden ways. Pulling this out of fantasy, we can think of the brilliant scientist who takes one step too far in his research and ends up opening a blackhole in the middle of his lab. Whoops.
Baltar would have designed if given the chance.
4. Treasure in Plain Sight
Players will debate about how smart the dungeon designer (especially a lich - see above) is. Are we talking the Sicilian from Princess Bride here? Maybe he put that gem out in plain sight so we'd grab it and instantly be teleported to the Room of Death that old man outside the entrance of the dungeon warned us about. Then again, what better way to protect your treasure from curious adventurers than to put it out where anyone can see it and create incredibly paranoid delusions about secret death gems?
Treasure, of course, is in the eye of the beholder. Where a kobold fighter wants desperately to find more gold to buy a bigger sword, modern day adventurers may need that piece of impossible-to-find information that will ruin their political enemy. What happens when they download all the files from the enemy's computer only to set off silent alarms within the system or secretly downloading a terrible virus that actually feeds your information to your enemy? You want to make the players trip over themselves to pick up the thing that will ruin them? Put something they can't resist right on top of it.
3. The Friendly Saboteur
The trick to pulling off The Friendly Saboteur is to make the character beyond convincing as an ally. It's a lot like what you see above with the Treasure in Plain Sight. If you're looking at a system like D&D, you're creating a character that has put all his points into charisma and bluff. Of course this only works if your players don't know that this is the fact.
Your players have to buy in to the fact that this kindly character they encounter is just another hapless travel like themselves. This involves some selling on your part, but you can't sell them too hard. If they want to walk away from the FS, then let them. If they want to kill the FS, of course let them try! Just be sure not to reveal the FS's secret motives and plan. The great part of the FS is, if played right, they end up leading the adventurers into more terrible situations to deal with. The FS is really an ancient of the Big Bad of the dungeon. He has knowledge of what lies ahead of the players and he'll gently direct them towards the less pleasant of two options if given the chance.
Of course if they do end up killing the FS, the players will never know 100% for certain that they haven't killed an innocent person. This is where the FS can really shine in a very Obi-Wan if-you-strike-me-down-I-shall-become-more-powerful-than-you-could-possibly-imagine kind of way. The death of the FS can birth distrust and doubt within the group. It can be the seed for the tree of ruin that'll grow right the the center of the party. It's pretty fun.
2. The Pit Trap
Really you need to wrap your mind around the idea that the dungeon - its basic architecture and structure - is itself a trap. Think back to the dark ages. Go ahead, I'll help: picture the rolling green pastures, the terrible, terrible smells, and the huge, imposing castles. Castles were to the dark ages as dungeons are to D&D. The entire design of a castle was to be a meat grinder. Think of the castle like a body. When the castle got sick - a.k.a. an invading force tried to take it over - it was incredibly skilled at deploying its antibodies - crossbowmen and spearmen - just where they need to be to kill off the infection. From murderholes to strategically-placed portcullis closing, a castle was made to keep invaders out and occupants alive.
The dungeon is the same way. Those pit traps weren't there by accident - they've been sitting in darkness since the very conception of the dungeon. There's a logic to them that the designer put a lot of work into. The logic could simply be the fact that's damn hard to predict where one might be in how randomly they seem placed, but hey, it's a strategy.
1. A Closed Door
Ogden Nash once said (or wrote, or whatever, dude was literally a poet) "A door is what a dog is perpetually on the wrong side of." I'd like to modify this a little bit and change dog to "adventurer."
While we're getting all literary here, let's break down the word adventurer. At it's core, an adventurer is one who seeks adventure. One definition of adventure I found that I think is particularly appropriate to role-playing: "take a risk in the hope of a favorable outcome." This is what we do and why we have dice in our games. The closed door represents every hope and every danger our characters may face in their journey to become whatever it is they want to be.
We have a joke in our group that the most terrifying dungeon we could ever encounter is a long hall way where a door is revealed every 15 feet. Whole adventures have come to a grinding halt thanks to a mysterious door appearing from no where. There's not going to be a lot of DM'ing advice on this one other than this: when you decided to put a door in a dungeon make sure there's something on the other side that makes it worth opening. Oh, also? Leave some of them unlocked - it makes the players super paranoid.
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