Similar to the OP, I also have a world with lots of necros (although mine was done via advanced world generation and took a bit longer):
Here's the template. Just edit your world_gen.txt file (found in data/init) and put this on the end of it:
[TITLE:NECROTIME]
[SEED:UEks8QS0QAyugWWOy8K4]
[HISTORY_SEED:sUAgMSM2WGecikeEki0m]
[NAME_SEED:k2i0IMAwqI6Au88u0KmG]
[CREATURE_SEED:qAIAAQU4MeCiecwmgWEY]
[DIM:33:33]
[EMBARK_POINTS:2500]
[END_YEAR:1000]
[BEAST_END_YEAR:1000:99]
[REVEAL_ALL_HISTORY:0]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:100:350:3200:3200]
[RAINFALL:10:65:0:0]
[TEMPERATURE:70:75:0:0]
[DRAINAGE:33:100:0:0]
[VOLCANISM:0:0:0:0]
[SAVAGERY:0:0:0:0]
[ELEVATION_FREQUENCY:1:9:0:0:0:1]
[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:2500]
[MEGABEAST_CAP:0]
[SEMIMEGABEAST_CAP:0]
[TITAN_NUMBER:0]
[TITAN_ATTACK_TRIGGER:0:0:0]
[DEMON_NUMBER:10]
[NIGHT_TROLL_NUMBER:10]
[BOGEYMAN_NUMBER:0]
[VAMPIRE_NUMBER:10]
[WEREBEAST_NUMBER:0]
[SECRET_NUMBER:500]
[REGIONAL_INTERACTION_NUMBER:0]
[DISTURBANCE_INTERACTION_NUMBER:500]
[EVIL_CLOUD_NUMBER:0]
[EVIL_RAIN_NUMBER:0]
[GOOD_SQ_COUNTS:0:0:0]
[EVIL_SQ_COUNTS:0:0:0]
[PEAK_NUMBER_MIN:0]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:0]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:1000]
[RIVER_MINS:0:0]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:0]
[SUBREGION_MAX:2750]
[CAVERN_LAYER_COUNT:1]
[CAVERN_LAYER_OPENNESS_MIN:1]
[CAVERN_LAYER_OPENNESS_MAX:2]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:0]
[CAVERN_LAYER_WATER_MIN:25]
[CAVERN_LAYER_WATER_MAX:25]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:5]
[LEVELS_ABOVE_LAYER_1:40]
[LEVELS_ABOVE_LAYER_2:2]
[LEVELS_ABOVE_LAYER_3:2]
[LEVELS_ABOVE_LAYER_4:2]
[LEVELS_ABOVE_LAYER_5:3]
[LEVELS_AT_BOTTOM:2]
[CAVE_MIN_SIZE:1]
[CAVE_MAX_SIZE:1]
[MOUNTAIN_CAVE_MIN:0]
[NON_MOUNTAIN_CAVE_MIN:0]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:100]
[TOTAL_CIV_POPULATION:1]
[SITE_CAP:1]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
I've modded goblins to reproduce and grow up faster, so there's more to toss into the meat grinder, along with making dragon eggs hatch, which is why I'm not uploading the world. Still, I tested it in vanilla, and the result is still the same on the necro front.
OP, have fun having multiple necromancer sieges! I've taken a break from the pictured world and am brushing up on my DF skills in a savage world. I tried embarking in range of nineteen towers (pictured below), and an ambushing necro made me panic. I sealed him in the fort with me. It didn't go well.