I have never seen such a thing, but I have just genned a world with one. What horrors await Mr. Beard and his wife, Doctor Winky?
You should see the place the dwarves have set up for themselves, though. Two dwarven civilization began in this place and radiated outward, through the mountains to the east and west. I'd upload, but modded races are involved.
It is surrounded by impassable mountains, and there is only one way in. To get in you'd have to fight past two dwarven mountainhomes (three if you count the volcano on the far right, which has a fort built on top of it), then a narrow path through jungle, and then there is a river you have to sail down, so I hope you carried some boats or something through the jungle. Not that it will help, since the river is populated by elves, who are allies with the dwarves it seems. Then there is ANOTHER DWARVEN FORT.
After all this, you can get to the innermost mountainhome. Which is across a river and also a fort. Also the elves have populated near every non-mountain tile within. Mind you that this covers only the major mountainhomes along the way, not the many smaller ones along the roads. And this is year 100.
These are high elves, by the way, who I modded in to be taller, more intellectual, less agile, and use metals. The only downside (?) is that it is hot and tropical. Guess they'll have to swim in their huge freshwater good-aligned lake within the valley to cool down. There's even flux and multiple metals. Here's a screen. That's the southern border of the world there, and it's quite impassable from the north, though the screen doesn't show it.
::EDIT::
Heres the worldgen. The evil volcano is in the northern half, the eastmost of a cluster of volcanoes.
[WORLD_GEN]
[TITLE:CUSTOM_DUNI]
[SEED:GM6iQEiSWCqoqoSY8ww0]
[HISTORY_SEED:KackO8E4yWoc6IEak8Mu]
[NAME_SEED:kusC4Gc4S0aOa0IW4KmA]
[CREATURE_SEED:IWq4YAyUqkse2MUso6oS]
[DIM:129:129]
[EMBARK_POINTS:1050]
[END_YEAR:100]
[BEAST_END_YEAR:200:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:401:401]
[RAINFALL:0:100:200:200]
[TEMPERATURE:25:75:200:200]
[DRAINAGE:0:100:200:200]
[VOLCANISM:0:100:200:200]
[SAVAGERY:0:100:200:200]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[MINERAL_SCARCITY:1000]
[MEGABEAST_CAP:30]
[SEMIMEGABEAST_CAP:60]
[TITAN_NUMBER:20]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:15]
[NIGHT_TROLL_NUMBER:10]
[BOGEYMAN_NUMBER:10]
[VAMPIRE_NUMBER:10]
[WEREBEAST_NUMBER:10]
[SECRET_NUMBER:10]
[REGIONAL_INTERACTION_NUMBER:28]
[DISTURBANCE_INTERACTION_NUMBER:28]
[EVIL_CLOUD_NUMBER:12]
[EVIL_RAIN_NUMBER:12]
[GOOD_SQ_COUNTS:25:251:503]
[EVIL_SQ_COUNTS:25:251:503]
[PEAK_NUMBER_MIN:12]
[PARTIAL_OCEAN_EDGE_MIN:1]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:10]
[REGION_COUNTS:SWAMP:260:1:1]
[REGION_COUNTS:DESERT:260:1:1]
[REGION_COUNTS:FOREST:1040:3:3]
[REGION_COUNTS:MOUNTAINS:2080:2:2]
[REGION_COUNTS:OCEAN:2080:1:1]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:2080:3:3]
[REGION_COUNTS:HILLS:2080:3:3]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:100:100]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:15]
[CAVE_MAX_SIZE:35]
[MOUNTAIN_CAVE_MIN:35]
[NON_MOUNTAIN_CAVE_MIN:60]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:40]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:1040]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:2080:4160:2080]
[RAIN_RANGES:2080:4160:2080]
[DRAINAGE_RANGES:2080:4160:2080]
[SAVAGERY_RANGES:2080:4160:2080]
[VOLCANISM_RANGES:2080:4160:2080]
One more thing. In the far northwest is some embark where you can have evil mountain and good forest in the same embark. enjoy.