In World Generation, the game would place a few Sleeping Beasts. You can see them on Embark Site when looking at site information, together to Deep Metals, Aquifer and stuff.
These beasts are extremely Fun, they re commonly so big that in game they re represented in more than one tile, and extremely hard to kill. They can be underground or at surface, or even in the bottom of oceans. They re sleeping, and inactive by normal. Every year there is a small chance it will wake up. It will also wake up when attacked or if noisy dwarves keep bothering it. The beast is randomized, similar to forgotten beast, but some common things in them is: They are always sleeping at beggining, they re giantic, they re extremely hard to kill, and if they re underground, they must have a way to reach surface, either by being able to dig onto blocks, and they re EXTREMELY, ABSOLUTELY, hostile.
They re so powerful, that, even when sleeping, they have effect on your dwarves life. Cave Ins re more common in zones with them as long as they re alive. If earthquakes are added, they may be extremely more common in zones with them, creatures will be scared of going near them.
Due to their size, most of your dwarves wouldnt do any real damage to them, even if they re legendary vampire warriors with extremely powerful weapons, and to deal any real damage to them, you will need siege weapons. They often have a lot of vital organs, for example, four hearts, thus making them die due to damage to vital organs harder.
Did i say already they re extremely Fun?