Here is an idea I wrote into my most recent campaign. Not entirely original since it was inspired by video games, but here goes.
Characters approach a door with 3 glowing runes above it (your choice of color). Tell the characters immediately (however you want to do it) that a mage or someone in the group gets a presence of great evil magic as they approach. The door is unlocked. The moment they enter the next room, the door locks shut behind them and on the other side (so in the room) are the same sets of runes in a different color above this door frame. The room dimensions are up to you, but if you have smart cleric player* in your group, make its spacious. The room contains the same runes on the doors but way larger on the floor (kinda like the seals from Diablo 2 in the "Chaos Sanctuary").
There is nothing of value in this room so the moment they touch the runes/seals on the ground, or attempt to leave, the runes/seals on the ground adopt the color of the runes from the inside and the runes on the inside door become the color of the door on the outside. Then 3 monsters of similar theme will simply walk out of one of the walls (i described them when they first entered as built in carvings into the wall like Han Solo in carbonite). The monsters should be scaling in intensity. So I used a zombie, a ghoul and a wight. The trick is they have to kill the monsters in the correct order (your choice) in order to unlock the room. Each time they downed a monster in the right order one of the floor runes will revert back to its original color and same with its corresponding locking rune on the inside of the door. The door will not open till all runes are the same color. The monsters will keep coming out of the walls once all three are down IF NOT killed in the right order. My players got frustrated because they couldn't figure it out so instead they got one down and the the wizard used 2 uses of the D&D Next "Knock" Spell to unlock the other two. He didn't think that each rune was a lock so he had to use 2. Each knock would null one rune.
My players had an average character lvl of 6 at this time so its not like they will get lots of xp from the monsters themselves; and the monsters are not tough if you fought just those 3 monsters, but the fact that it doesn't end till they leave or die is interesting. No short rests, nothing. Instead I granted them more exp for actually getting out. I had a fixed exp I was going to give them regardless of how they got out.
I apologize if this is confusing, feel free to PM me if you have further questions/ deeper explanation/ want to know more about my current adventure "Beyond the Abyss".
*if you have a smart cleric, and you used undead themed monsters, he/she will try to "turn undead" . Make sure you stress that (at least in D&D Next), turn undead has an area of effect and monsters must be in that area. Similarly if u decide the order is wight, zombie, ghoul, if they turn undead and they all get destroyed instead of turned, the closest one to the cleric dies first- if they don't die in the right order, cool they are obliterated by holy light, but 3 more will come right out again. Don't let them try to pull a fast one, but reward creativity and out of the box thinking.
