I developed a Mindflayer sieger race for use in Dwarf Fortress.
Main Features:
1. They wield weapons in their tentacles, can eat your brains, and will eat enough brains of your children to brainwash them into their military.
2. They use the "elegant" weapons, bows, swords, pikes, and maces, with the occasional whip or scourge.
3. They will Thieve and Babysnatch, as well as siege. Their better soldiers will be equipped with Steel, and are just as dangerous without weapons as with them.
4. They are very very intelligent, and very very brutal. They can and will pick your locks and avoid your traps.
I've done a good amount of testing (Both in world gen and Fortress mode), and I can attest that they work quite nicely.
So, here's how to add them:
Add this to the creature_standard file in the raws:
[CREATURE:MINDFLAYER]
[NAME:mindflayer:mindflayers:mindflayer]
[TILE:'F'][COLOR:66:49:137]
[GLOWTILE:'%'][GLOWCOLOR:4:0:1]
[GENPOWER:2]
[EVIL]
[INTELLIGENT][liKES_FIGHTING]
[NOPAIN][NOSTUN][NONAUSEA][NOEMOTION]
[BLOODTYPE:C]
[BONECARN]
[POWER]
[trAPAVOID]
[LOCKPICKER]
[CANOPENDOORS]
[EXTRAVISION]
[trANCES]
[PREFSTRING:alien features]
[PREFSTRING:psychic powers]
[PREFSTRING:brain eating]
[BODY:HUMANOID:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:5FINGERS:5TOES:MOUTH:SIX_TENTACLES_HEAD]
[ATTACK:MAIN:BYTOKEN:SIX_TENTACLES_HEAD:rend:rends:6:6:GORE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:50:100][ATTACKFLAG_WITH]
[CHILD:12][BABY:1]
[CHILDNAME:tadpole:tadpoles]
[KING_NAME:Brain King:Brain Kings]
[BARON_NAME:Thrall Lord:Thrall Lords]
[HIGH_PRIEST_NAME:Thrall Herd:Thrall Herds]
[SIZE:7]
[FAT:2]
[EQUIPS]
[NOCTURNAL]
[STANDARD_FLESH]
[HOMEOTHERM:10067]
[LAYERING:10]
[SWIMS_LEARNED][SWIM_SPEED:2500]
[PERSONALITY:ANGER:0:0:0]
[PERSONALITY:ARTISTIC_INTEREST:0:75:100]
[PERSONALITY:ASSERTIVENESS:0:50:100]
[PERSONALTIY:DUTIFULNESS:50:75:100]
[PERSONALITY:IMAGINATION:25:50:100]
[PERSONALITY:IMMODERATION:0:0:0]
[PERSONALTIY:INTELLECTUAL_CURIOSITY:100:100:100]
[PERSONALITY:ORDERLINESS:100:100:100]
[PERSONALITY:SELF_DISCIPLINE:100:100:100]
[PERSONALITY:EMOTIONALITY:0:0:0]
[PERSONALITY:ACHIEVEMENT_STRIVING:100:100:100]
[PERSONALITY:EXCITEMENT_SEEKING:0:0:0]
[PERSONALITY:ACTIVITY_LEVEL:80:90:100]
[PERSONALITY:CHEERFULNESS:0:0:0]
[PERSONALITY:ALTRUISM:0:0:0]
[PERSONALITY:SYMPATHY:0:0:0]
[PERSONALITY:VULNERABILITY:0:0:0]
[PERSONALITY:COOPERATION:0:50:100]
Add this to the entity_default file, also in the raws
[ENTITY:ENCLAVE]
[CREATURE:MINDFLAYER]
[trANSLATION:GOBLIN]
[DIGGER:ITEM_WEAPON_PICK]
[WEAPON:ITEM_WEAPON_WHIP]
[WEAPON:ITEM_WEAPON_SWORD_SHORT]
[WEAPON:ITEM_WEAPON_SPEAR]
[WEAPON:ITEM_WEAPON_MACE]
[WEAPON:ITEM_WEAPON_BOW]
[AMMO:ITEM_AMMO_ARROWS]
[WEAPON:ITEM_WEAPON_SWORD_LONG]
[WEAPON:ITEM_WEAPON_SCOURGE]
[WEAPON:ITEM_WEAPON_FLAIL]
[WEAPON:ITEM_WEAPON_MORNINGSTAR]
[WEAPON:ITEM_WEAPON_SCIMITAR]
[ARMOR:ITEM_ARMOR_PLATEMAIL:COMMON]
[ARMOR:ITEM_ARMOR_CHAINMAIL:COMMON]
[ARMOR:ITEM_ARMOR_LEATHER:COMMON]
[ARMOR:ITEM_ARMOR_CLOAK:COMMON]
[ARMOR:ITEM_ARMOR_TUNIC:COMMON]
[ARMOR:ITEM_ARMOR_ROBE:COMMON]
[GLOVES:ITEM_GLOVES_GAUNTLETS:COMMON]
[GLOVES:ITEM_GLOVES_GLOVES:COMMON]
[SHOES:ITEM_SHOES_SHOES:COMMON]
[SHOES:ITEM_SHOES_BOOTS:COMMON]
[SHOES:ITEM_SHOES_BOOTS_LOW:COMMON]
[SHOES:ITEM_SHOES_SOCKS:COMMON]
[PANTS:ITEM_PANTS_PANTS:COMMON]
[PANTS:ITEM_PANTS_GREAVES:COMMON]
[PANTS:ITEM_PANTS_LEGGINGS:COMMON]
[SHIELD:ITEM_SHIELD_SHIELD]
[SHIELD:ITEM_SHIELD_BUCKLER]
[SIEGEAMMO:ITEM_SIEGEAMMO_BALLISTA]
[TOY:ITEM_PUZZLEBOX]
[trAPCOMP:ITEM_TRAPCOMP_GIANTAXEBLADE]
[trAPCOMP:ITEM_TRAPCOMP_ENORMOUSCORKSCREW]
[trAPCOMP:ITEM_TRAPCOMP_SPIKEDBALL]
[trAPCOMP:ITEM_TRAPCOMP_LARGESERRATEDDISC]
[trAPCOMP:ITEM_TRAPCOMP_MENACINGSPIKE]
[INSTRUMENT:ITEM_INSTRUMENT_FLUTE]
[INSTRUMENT:ITEM_INSTRUMENT_HARP]
[CLOTHING]
[SUBTERRANEAN_CLOTHING]
[CURRENCY:COPPER:1]
[CURRENCY:SILVER:5]
[CURRENCY:GOLD:15]
[SELECT_SYMBOL:WAR:NAME_WAR]
[SUBSELECT_SYMBOL:WAR:VIOLENT]
[SELECT_SYMBOL:BATTLE:NAME_BATTLE]
[SUBSELECT_SYMBOL:BATTLE:VIOLENT]
[SELECT_SYMBOL:SIEGE:NAME_SIEGE]
[SUBSELECT_SYMBOL:SIEGE:VIOLENT]
[SELECT_SYMBOL:ROAD:NAME_ROAD]
[SELECT_SYMBOL:TUNNEL:NAME_TUNNEL]
[SELECT_SYMBOL:BRIDGE:NAME_BRIDGE]
[SELECT_SYMBOL:WALL:NAME_WALL]
[SELECT_SYMBOL:REMAINING:EVIL]
[CULL_SYMBOL:ALL:DOMESTIC]
[CULL_SYMBOL:ALL:FLOWERY]
[CULL_SYMBOL:ALL:HOLY]
[CULL_SYMBOL:ALL:PEACE]
[CULL_SYMBOL:ALL:NEGATOR]
[CULL_SYMBOL:ALL:GOOD]
[CULL_SYMBOL:ALL:UGLY]
[STONE_PREF]
[METAL_PREF]
[USE_CAVE_ANIMALS]
[USE_EVIL_ANIMALS]
[USE_EVIL_PLANTS]
[USE_EVIL_WOOD]
[USE_ANIMAL_PRODUCTS]
[USE_MISC_PROCESSED_WOOD_PRODUCTS]
[EQUIPMENT_IMPROVEMENTS]
[ABUSE_BODIES]
[SPHERE_ALIGNMENT:WAR:512]
[ART_FACET_MODIFIER:GOOD:0]
[ART_FACET_MODIFIER:EVIL:512]
[FRIENDLY_COLOR:5:0:1]
[DEFAULT_SITE_TYPE:DARK_FORTRESS]
[liKES_SITE:DARK_FORTRESS]
[liKES_SITE:DETAILED_CAVE]
[liKES_SITE:RUIN]
[TOLERATES_SITE:DETAILED_CAVE]
[TOLERATES_SITE:RUIN]
[TOLERATES_SITE:DARK_FORTRESS]
[CAN_HAVE_MILITARY_SITE_LEADER]
[START_BIOME:MOUNTAIN]
[BIOME_SUPPORT:ANY_WETLAND:1]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:ANY_FOREST:2]
[BIOME_SUPPORT:ANY_OCEAN:1]
[BIOME_SUPPORT:ANY_LAKE:1]
[BIOME_SUPPORT:MOUNTAIN:3]
[BIOME_SUPPORT:ANY_GRASSLAND:1]
[BIOME_SUPPORT:ANY_SAVANNA:1]
[BIOME_SUPPORT:ANY_SHRUBLAND:1]
[BIOME_SUPPORT:ANY_RIVER:1]
[PROGRESS_TRIGGER_POPULATION:4]
[PROGRESS_TRIGGER_PRODUCTION:4]
[PROGRESS_TRIGGER_TRADE:4]
[ACTIVE_SEASON:SPRING]
[ACTIVE_SEASON:SUMMER]
[ACTIVE_SEASON:AUTUMN]
[ACTIVE_SEASON:WINTER]
[BABYSNATCHER]
[MAX_STARTING_CIV_NUMBER:100]
[START_GROUP_NUMBER:10]
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
[BEAST_HUNTER]
[SCOUT]
[PERMITTED_JOB:MINER]
[PERMITTED_JOB:CARPENTER]
[PERMITTED_JOB:BOWYER]
[PERMITTED_JOB:WOODCUTTER]
[PERMITTED_JOB:ENGRAVER]
[PERMITTED_JOB:MASON]
[PERMITTED_JOB:ANIMAL_CARETAKER]
[PERMITTED_JOB:ANIMAL_TRAINER]
[PERMITTED_JOB:HUNTER]
[PERMITTED_JOB:TRAPPER]
[PERMITTED_JOB:ANIMAL_DISSECTOR]
[PERMITTED_JOB:FURNACE_OPERATOR]
[PERMITTED_JOB:WEAPONSMITH]
[PERMITTED_JOB:ARMORER]
[PERMITTED_JOB:BLACKSMITH]
[PERMITTED_JOB:METALCRAFTER]
[PERMITTED_JOB:GEM_CUTTER]
[PERMITTED_JOB:GEM_SETTER]
[PERMITTED_JOB:WOODCRAFTER]
[PERMITTED_JOB:STONECRAFTER]
[PERMITTED_JOB:LEATHERWORKER]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:WEAVER]
[PERMITTED_JOB:CLOTHIER]
[PERMITTED_JOB:GLASSMAKER]
[PERMITTED_JOB:FISHERMAN]
[PERMITTED_JOB:FISH_DISSECTOR]
[PERMITTED_JOB:FISH_CLEANER]
[PERMITTED_JOB:COOK]
[PERMITTED_JOB:BUTCHER]
[PERMITTED_JOB:TANNER]
[PERMITTED_JOB:DYER]
[PERMITTED_JOB:HERBALIST]
[PERMITTED_JOB:BREWER]
[PERMITTED_JOB:LYE_MAKER]
[PERMITTED_JOB:WOOD_BURNER]
[PERMITTED_JOB:MECHANIC]
[PERMITTED_JOB:SIEGE_ENGINEER]
[PERMITTED_JOB:SIEGE_OPERATOR]
[PERMITTED_JOB:PUMP_OPERATOR]
[PERMITTED_JOB:TRADER]
[PERMITTED_JOB:ARCHITECT]
[WORLD_CONSTRUCTION:TUNNEL]
[WORLD_CONSTRUCTION:BRIDGE]
[ETHIC:KILL_ENTITY_MEMBER:PERSONAL_MATTER]
[ETHIC:KILL_NEUTRAL:ACCEPTABLE]
[ETHIC:KILL_ENEMY:ACCEPTABLE]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:ACCEPTABLE]
[ETHIC:TORTURE_FOR_INFORMATION:ACCEPTABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:ACCEPTABLE]
[ETHIC:TREASON:PUNISH_CAPITAL]
[ETHIC:OATH_BREAKING:PUNISH_CAPITAL]
[ETHIC:LYING:PUNISH_CAPITAL]
[ETHIC:VANDALISM:PUNISH_CAPITAL]
[ETHIC:TRESPASSING:PUNISH_CAPITAL]
[ETHIC:THEFT:PUNISH_CAPITAL]
[ETHIC:ASSAULT:PERSONAL_MATTER]
[ETHIC:SLAVERY:ACCEPTABLE]
[ETHIC:EAT_SAPIENT_OTHER:ACCEPTABLE]
[ETHIC:EAT_SAPIENT_KILL:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:ACCEPTABLE]
And add this to the body_default page
[BODY:SIX_TENTACLES_HEAD]
[BP:RUT:right upper tentacle][CON:HD][liMB]
[BP:LUT:left upper tentacle][CON:HD][liMB]
[BP:RUTC:right upper claw][CON:RUT]
[BP:LUTC:left upper claw][CON:LUT]
[BP:RMT:right middle tentacle][CON:HD][liMB]
[BP:LMT:left middle tentacle][CON:HD][liMB]
[BP:RMTC:right middle claw][CON:RMT]
[BP:LMTC:left middle claw][CON:LMT]
[BP:RLT:right lower tentacle][CON:HD][liMB]
[BP:LLT:left lower tentacle][CON:HD][liMB]
[BP:RLTC:right lower claw][CON:RLT]
[BP:LLTC:left lower claw][CON:LLT]
I was considering writing them a new language, but I'd like to get some input before I got as far as that.
After you add this, you will have to gen a new world, as that's the only way to add a new civilization.