There is a lot of ruby dust and emerald dust to be found on level 13, if the characters take time to gather it. It looks as if whoever was doing the building on level 12 swept it (and a lot of other junk) down to level 13.
If the player characters get as far as this level, start making serious attempts to kill them.
The resident monsters on this level do not cower away in corners. They are big enough and nasty enough to take care of themselves.
If you have a favourite nasty, someone or something scary enough to frighten gargoyles and trolls, please feel free to include him, her or it as a resident monster of level 13. If your favourite nasty has henchcreatures, include them on the wandering monster table.
A few suggestions:
It is about time that the player characters encountered a real vampire. That is, a vampire who used to be human and who is, in addition to being a vampire, also a high-level cleric and/or mage. The vampire will have plenty of slave semi-vampires to use as cannon-fodder against player characters.
Spectral medusas are also fun. Assume that they do not reflect in mirrors, so cannot petrify themselves accidentally.
Grey dragons are another possibility. Perhaps you have not come across grey dragons before. They are evil (possibly neutral evil), they breathe a petrification gas, and they can assume the form of gargoyles when they need to move around human-sized mazes. Their favourite food is petrified player character. Other than that, they are much the same as any other sort of evil dragon.
Or you could always fall back on that perennial favourite, the high-level evil wizard. Remember: the wizard plays to win. He does not stay alive on level 13 by being trusting and generous. Casting timestop and slaying the entire party is perfectly fair. They should have known better than all to be within range at the same time.
Whenever you feel the time is right, roll 2d6.
| 2d6 | Monster |
| 2 | Semivampires |
| 3 | Supergargoyles |
| 4 | Shadows |
| 5 | Gargoyles (+ Supergargoyle) |
| 6 | Wraiths |
| 7 | Soldiers |
| 8 | Ogres |
| 9 | Trolls |
| 10 | Hill Giant |
| 11 | Stone Giant |
| 12 | Medusa |
Wandering monsters are there to annoy, not to kill the entire party (that's what the resident monsters are for). Choose the numbers accordingly.
Remember also to include as many oozes, slimes, gelatinous cubes and other gooey things as you like, as well as enough carrion crawlers and other clean-up monsters to make sure that anything left lying around is not left lying around for too long. Keep your dungeon tidy!