I have totally updated all demons to v0.34, and improved a few demons, as well as adding 40d demons.
They should all work as intended.
SPIRIT OF FIRE- Straight back from 40d, these are humanoids made of flame that breath dragonfire. Extremely fragile, fast, great skills, can fly, and hotter than the surface of the sun ! (May be a bit weak due to being a total glass cannon)
OBSIDIAN COLOSSUS- Larger than bronze colossi, these rocky monstruosities will claw, punch, wrestle and kick. They can and will also breathe magma (liquid obsidian) although you won't see it accumulating on Hell's floor as 1/7 tiles, it certainly burns as well as fluid magma ! Luckily for you, they cannot fly. (By far the most dangerous demon)
DEVILKIN- Small, quick to flee and only brave in numbers, these small imp-like demons happen in very large numbers. They are mostly flying, intelligent, more aggressive and more skilled crundles. (Mostly a grand annoyance, although it can prove more than that...)
NIGHTMARE- A larger, fire-immune black horse, with a flaming mane and hooves. Happen in medium numbers, and can fly. Average skill. Quite fast and aggressive, prone to frequent rage, although they are generally no match for good soldiers. They do not breathe fire, but may set fire to things. (Not much of a threat if you have a decent enough military, prepared)
HELL HOUND- Four times larger than the common dog and fire-breathing, these infernal canines will happen in quite large numbers. They do not fly, but are very aggressive and tend to rage often. (Decent threat level, I guess)
BEBILITH- These colossal spiders are very fast, spin and throw webs, and can stab their enemies with their sharp front legs. They are 11 times the size of a regular giant cave spider, and are intelligent and skilled enough to pose a threat. Their venom is necrotic and quite dangerous. They also ambush your dwarves, but cannot fly. Just like most spiders they are extremely territorial and only happen in single individuals. (May not be as dangerous as it sounds. I may buff it in the future)
STRIGE- Not enough annoying enemies ? Here's another one ! They are skinless mosquitoes as big as a large dog. They have no fear, feel no emotion, are quite fast and are just extremely annoying. While slightly naturally skilled, they are mindless beasts, unlike other demons, and they do not get better at fighting with time. They tend to arrive and attack in very large groups, although not as large as devilkin. (Tested them back then in 0.31.25 against my legendary dwarves. They're a pain in the ass)
FROST WRAITH- They are flying gargoyles made of ice, and as large as a bear - and nearly every part of their body is a weapon ! They also breathe frostbite-inducing freezing clouds, although the efficiency of these tends to be mixed. They are also very cold at touch and immune to cold damage. Skilled enough, and comes in mid-sized groups. (Not much of a threat, although their breath weapon may be)
FROG DEMON- And another fiend back from 40d ! They are humanoid frogs, non-fliers, twice as big as a giant toad and far faster than one (although still slow by dwarf standards). While not very damaging nor very resistant, their main weapon is the noxious, foul-smelling oil their skin secretes, and this oil can temporarily incapacitate a dwarf warrior, even if legendary. They also happen in mid-sized groups and cannot fly. Do NOT let them go into your reserves of water ! (For a demon it's not dangerous without the syndrome - quite a bit more with)
TENTACLE DEMON- And for the last 40d demon... the infamous tentacle demon ! These have the basic form of an human with six tentacles attached, with a claw on each tentacle. They are as big as a giant cave spider and have corrupt intentions. They are able to emit a soporific, willpower-draining gas, and their claws exsude a venom that is nonlethal as well, but has annoying short-length consequences and permanently cripple some of your dwarf's mental attributes. They cannot fly and happen in mid-sized groups. Also, don't let your children near them ! (Not very dangerous, excepted for their syndromes)