Started with a bit of modding today. Custom fortress defense mod, with allies, enemies, thieves and cave-civs. I made 15 each, in 5 different difficulty settings. All use vanilla DF creatures as basis, so that players don't have to learn about made-up stuff by me. ^^
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Allies - These civs act like your human allies. They send caravans and trade with you; a small chance of war remains.
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TOAD_MAN
PENGUIN MAN
SLUG_MAN
DESERT_TORTOISE_MAN
BEETLE_MAN
GOAT_MOUNTAIN_MAN
GNOME_MOUNTAIN
PLATYPUS MAN
PANDA MAN
GAZELLE_MAN
ARMADILLO MAN
MUSKOX_MAN
CHINCHILLA_MAN
CHAMELEON_MAN
AXOLOTL_MAN
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Enemies - These civs act like goblins. Hostile to your dwarves, they send ambushes, snatchers and sieges to break your fort.
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Level I - No weapons, no armor. Siege very early.
MANTIS_MAN
RAT_MAN
ROACH_MAN
Level II - Wooden weapons, wooden armor. Sieges early. Sends ambushes.
BEAR_GRIZZLY_MAN
BROWN_RECLUSE_SPIDER_MAN
WOLVERINE_MAN
Level III - Bronze weapons, bronze armor. Sieges as usual. Brings cave animals.
BADGER MAN
LION_MAN
WALRUS_MAN
Level IV - Iron weapons, iron armor. Sieges late. Brings cave and evil animals. Doesn't fight with wildlife.
TIGER_MAN
ALLIGATOR_MAN
LIZARD_MAN
Level V - Steel weapons, steel armor. Sieges very late. Brings selection of any possible animals. Doesn't fight with wildlife and ignore neutrals. Might wait you out, besieging your fort till you attack.
BLENDEC_FOUL
ELEPHANT_MAN
BLIZZARD_MAN
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Thieves - Acting like kobolds, these civs will send single individuals to steal objects from your fort. Once they have stolen three items, they might attack you in force! They often hide in caves and don't appear in the world-gen menu.
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Level I - No weapons, no armor. Steal very early.
WORM_MAN
KEA_MAN
GNOME_DARK
Level II - Wooden weapons, wooden armor. Steal early.
FOX_MAN
RACCOON_MAN
HONEY BADGER MAN
Level III - Copper weapons, copper armor. Steals as usual.
GREMLIN
SATYR
GRIMELING
Level IV - Iron weapons, iron armor. Steals late.
RATTLESNAKE_MAN
BLACK_MAMBA_MAN
BLIZZARD_MAN
Level V - Steel weapons, steel armor. Steals very late.
AARDVARK_MAN
AYE-AYE_MAN
KING_COBRA_MAN
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Caverns - These tiny civs don't siege, steal or trade. They only appear in small groups of hunter-gatherers in the caverns below.
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Level I - Wooden weapons, no armor.
PLUMP_HELMET_MAN
REACHER
GORLAK
Level II - Wooden weapons, wooden shields.
ELEMENTMAN_GABBRO
ELEMENTMAN_AMETHYST
BLOOD_MAN
Level III - Bronze weapons, bronze armor.
VORACIOUS_CAVE_CRAWLER
MOLEMARIAN
ELEMENTMAN_MUD
Level IV - Iron weapons, Iron armor.
BLIND_CAVE_OGRE
TROGLODYTE
IMP_FIRE
Level V - Steel weapons, Steel armor.
ELEMENTMAN_IRON
ELEMENTMAN_MAGMA
ELEMENTMAN_FIRE