What are the unmentionable enemies in Dwarf Fortress?

My understanding is that if you dig down deep enough in Dwarf Fortress, you can run into an extraordinarily nasty type of monster. It also seems to be a tradition among players of the game not to mention these monsters' names on forums — they're too evil, like Voldemort.

I don't have the game, but my curiosity has gotten the better of me. What are these enemies called? Why are they so awful (both from how they fit into the lore and how they affect gameplay)?

Spoiler - highlight with your cursor if you want to spoil your Dwarven fun.

There are great enemies lurking at the bottom of the mineable stone depths. They are collectively refereed to as "demons". Forum tradition has them often referred to as clowns, and the place where they are imprisoned (Hell or sometimes Hidden Fun Stuff) as the circus (as to avoid spoiling the player's fun). They cannot access the player's realm unless the player mines the adamantine spires. Adamantine is often referred to as cotton candy. Also, demon species are randomly generated as a combination of names and features animals have. As such, no two worlds will feature the same kind of demons.

As for why those enemies are awful:

As soon as an adamantine spire breaches Hell, hundreds of demons start to spawn below it. If a player doesn't lock that part of the fortress, demons will quickly overrun and slaughter every living being in the fortress. Also, and in comparison with the average dwarf, their stats are far higher, making them not only more deathly but more difficult to kill and/or to trap.