They are kind of like elves, except they are evil and don't respect nature as much. I just copied the dwarf entity and made it elfy with a touch of evil. Since they are evil, I'm not sure how it will affect caravans, you'll probably wind up trading with goblins and kobolds.
paste this into the top of your entity_default.txt in your raws, above dwarves:
[ENTITY:DARK_ELF]
[CIV_CONTROLLABLE]
[CREATURE:DARK_ELF]
[TRANSLATION:ELF]
[DIGGER:ITEM_WEAPON_PICK]
[WEAPON:ITEM_WEAPON_AXE_BATTLE]
[WEAPON:ITEM_WEAPON_HAMMER_WAR]
[WEAPON:ITEM_WEAPON_SWORD_SHORT]
[WEAPON:ITEM_WEAPON_SPEAR]
[WEAPON:ITEM_WEAPON_MACE]
[WEAPON:ITEM_WEAPON_CROSSBOW]
[AMMO:ITEM_AMMO_BOLTS]
[WEAPON:ITEM_WEAPON_BOW]
[AMMO:ITEM_AMMO_ARROWS]
[WEAPON:ITEM_WEAPON_WHIP]
[WEAPON:ITEM_WEAPON_HALBERD]
[WEAPON:ITEM_WEAPON_SWORD_2H]
[WEAPON:ITEM_WEAPON_SWORD_LONG]
[WEAPON:ITEM_WEAPON_SCOURGE]
[WEAPON:ITEM_WEAPON_FLAIL]
[WEAPON:ITEM_WEAPON_MORNINGSTAR]
[ARMOR:ITEM_ARMOR_PLATEMAIL:COMMON]
[ARMOR:ITEM_ARMOR_CHAINMAIL:COMMON]
[ARMOR:ITEM_ARMOR_LEATHER:COMMON]
[ARMOR:ITEM_ARMOR_COAT:COMMON]
[ARMOR:ITEM_ARMOR_SHIRT:COMMON]
[ARMOR:ITEM_ARMOR_CLOAK:COMMON]
[ARMOR:ITEM_ARMOR_TUNIC:COMMON]
[ARMOR:ITEM_ARMOR_TOGA:UNCOMMON]
[ARMOR:ITEM_ARMOR_VEST:COMMON]
[ARMOR:ITEM_ARMOR_DRESS:COMMON]
[ARMOR:ITEM_ARMOR_ROBE:COMMON]
[HELM:ITEM_HELM_HELM:COMMON]
[HELM:ITEM_HELM_CAP:COMMON]
[HELM:ITEM_HELM_HOOD:COMMON]
[GLOVES:ITEM_GLOVES_GAUNTLETS:COMMON]
[GLOVES:ITEM_GLOVES_GLOVES:COMMON]
[GLOVES:ITEM_GLOVES_MITTENS: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]
[TRAPCOMP:ITEM_TRAPCOMP_GIANTAXEBLADE]
[TRAPCOMP:ITEM_TRAPCOMP_ENORMOUSCORKSCREW]
[TRAPCOMP:ITEM_TRAPCOMP_SPIKEDBALL]
[TRAPCOMP:ITEM_TRAPCOMP_LARGESERRATEDDISC]
[TRAPCOMP:ITEM_TRAPCOMP_MENACINGSPIKE]
[TOY:ITEM_TOY_PUZZLEBOX]
[TOY:ITEM_TOY_BOAT]
[TOY:ITEM_TOY_BOW]
[TOY:ITEM_TOY_VOODOO]
[TOY:ITEM_TOY_IDOL]
[INSTRUMENT:ITEM_INSTRUMENT_FLUTE]
[INSTRUMENT:ITEM_INSTRUMENT_OCARINA]
[INSTRUMENT:ITEM_INSTRUMENT_HARP]
[INSTRUMENT:ITEM_INSTRUMENT_VIOLIN]
[INSTRUMENT:ITEM_INSTRUMENT_PICCOLO]
[CLOTHING]
[SUBTERRANEAN_CLOTHING]
[CURRENCY_BY_YEAR]
[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:NATURE]
[SELECT_SYMBOL:REMAINING:FLOWERY]
[CULL_SYMBOL:ALL:DOMESTIC]
[CULL_SYMBOL:ALL:SUBORDINATE]
[CULL_SYMBOL:ALL:EVIL]
[CULL_SYMBOL:ALL:UNTOWARD]
[CULL_SYMBOL:ALL:FLOWERY]
[CULL_SYMBOL:ALL:NEGATIVE]
[CULL_SYMBOL:ALL:UGLY]
[CULL_SYMBOL:ALL:NEGATOR]
[METAL_PREF]
[GEM_PREF]
[STONE_PREF]
[OUTDOOR_FARMING]
[USE_CAVE_ANIMALS]
[USE_EVIL_ANIMALS]
[USE_ANIMAL_PRODUCTS]
[COMMON_DOMESTIC_PACK]
[COMMON_DOMESTIC_PULL]
[COMMON_DOMESTIC_MOUNT]
[COMMON_DOMESTIC_PET]
[USE_MISC_PROCESSED_WOOD_PRODUCTS]
[EQUIPMENT_IMPROVEMENTS]
[ART_IMAGE_ELEMENT_MODIFIER:PLANT:64]
[ART_IMAGE_ELEMENT_MODIFIER:TREE:64]
[ITEM_IMPROVEMENT_MODIFIER:RINGS_HANGING:64]
[ITEM_IMPROVEMENT_MODIFIER:BANDS:384]
[ITEM_IMPROVEMENT_MODIFIER:SPIKES:384]
[ADVENTURE_TIER:3]
[FRIENDLY_COLOR:1:0:1]
[DEFAULT_SITE_TYPE:TREE_CITY]
[LIKES_SITE:TREE_CITY]
[TOLERATES_SITE:CITY]
[TOLERATES_SITE:TREE_CITY]
[TOLERATES_SITE:CAVE_DETAILED]
[TOLERATES_SITE:DARK_FORTRESS]
[LEADER_TYPE:DRUID]
[SITE_LEADER_TYPE:DRUID]
[START_BIOME:ANY_FOREST]
[BIOME_SUPPORT:ANY_LAKE:1]
[BIOME_SUPPORT:ANY_FOREST:3]
[BIOME_SUPPORT:ANY_RIVER:1]
[DIPLOMAT]
[DIPLOMAT_BODYGUARDS]
[MERCHANT_BODYGUARDS]
[ACTIVE_SEASON:AUTUMN]
[SIEGER]
[MAX_STARTING_CIV_NUMBER:100]
[START_GROUP_NUMBER:10]
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
[RELIGION:ANY_APPROPRIATE_POWER]
[MAYOR]
[WANDERER]
[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:STRAND_EXTRACTOR]
[PERMITTED_JOB:FISHERMAN]
[PERMITTED_JOB:FISH_DISSECTOR]
[PERMITTED_JOB:FISH_CLEANER]
[PERMITTED_JOB:CHEESE_MAKER]
[PERMITTED_JOB:MILKER]
[PERMITTED_JOB:COOK]
[PERMITTED_JOB:THRESHER]
[PERMITTED_JOB:MILLER]
[PERMITTED_JOB:BUTCHER]
[PERMITTED_JOB:TANNER]
[PERMITTED_JOB:DYER]
[PERMITTED_JOB:PLANTER]
[PERMITTED_JOB:HERBALIST]
[PERMITTED_JOB:BREWER]
[PERMITTED_JOB:SOAP_MAKER]
[PERMITTED_JOB:POTASH_MAKER]
[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:CLERK]
[PERMITTED_JOB:ADMINISTRATOR]
[PERMITTED_JOB:TRADER]
[PERMITTED_JOB:ARCHITECT]
[ETHIC:KILL_ENTITY_MEMBER:PUNISH_CAPITAL]
[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:UNTHINKABLE]
[ETHIC:TREASON:PUNISH_CAPITAL]
[ETHIC:OATH_BREAKING:PUNISH_CAPITAL]
[ETHIC:LYING:PERSONAL_MATTER]
[ETHIC:VANDALISM:PUNISH_SERIOUS]
[ETHIC:TRESPASSING:PUNISH_SERIOUS]
[ETHIC:THEFT:PUNISH_SERIOUS]
[ETHIC:ASSAULT:PUNISH_SERIOUS]
[ETHIC:SLAVERY:ACCEPTABLE]
[ETHIC:EAT_SAPIENT_OTHER:ACCEPTABLE]
[ETHIC:EAT_SAPIENT_KILL:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:APPALLING]
[ETHIC:MAKE_TROPHY_SAPIENT:SHUN]
[ETHIC:MAKE_TROPHY_ANIMAL:ACCEPTABLE]
[WILL_ACCEPT_TRIBUTE]
and paste this into your creature_standard.txt:
[CREATURE:DARK_ELF]
[NAME:Dark_Elf:Dark_Elves:Dark_Elven]
[TILE:'E'][COLOR:3:0:0]
[SPEED:700][GRASSTRAMPLE:0]
[GENPOWER:3]
[INTELLIGENT]
[CANOPENDOORS]
[EVIL]
[PREFSTRING:grace]
[BODY:HUMANOID:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:5FINGERS:5TOES:MOUTH]
[NARROW]
[DAMBLOCK:-1]
[SIZE:7]
[ATTACK:MAIN:BYTYPE:GRASP:punch:punches:1:2:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:SECOND:BYTYPE:MOUTH:bite:bites:1:1:GORE][ATTACKFLAG_CANLATCH]
[CHILD:12][BABY:1][MULTIPLE_LITTER_RARE]
[FAT:1]
[EQUIPS]
[NOCTURNAL]
[CRAFTSMAN_NAME:craftself:craftselves]
[FISHERMAN_NAME:fisherelf:fisherelves]
[HAMMERMAN_NAME:hammerelf:hammerelves]
[SPEARMAN_NAME:spearelf:spearelves]
[CROSSBOWMAN_NAME:markselff:markselves]
[AXEMAN_NAME:axedwarf:axe_elves]
[SWORDSMAN_NAME:swordself:swordselves]
[MACEMAN_NAME:mace_elf:mace_elves]
[PIKEMAN_NAME:pike_elf:pike_elves]
[BOWMAN_NAME:bowelf:bowdelves]
[SPEECH:elf.txt]
[STANDARD_FLESH]
[HOMEOTHERM:10067]
[SWIMS_LEARNED][SWIM_SPEED:2500]
[PERSONALITY:IMAGINATION:0:55:100]
[PERSONALITY:ARTISTIC_INTEREST:0:60:100]
[PERSONALITY:INTELLECTUAL_CURIOSITY:0:55:100]
[PERSONALITY:SELF_DISCIPLINE:0:45:100]
[PERSONALITY:ACTIVITY_LEVEL:0:40:100]
Make sure to generate a new world with these mods, you can't use a previously generate one. Also, one weird thing, and I'm not sure if this is something that happens normally and I just noticed or if my mods are doing this, but there were a few dark dwarf and dark human settlements in world gen. Anyway, I have successfully embarked as Dark Elves, but I haven't gone much farther than that yet. I have no idea if this will work in 2010.