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In the deep, there are beasts so fell and terrible, that only they know what they are, for none who have met them have lived to tell of it... they are the Forgotten Beasts, born of the chaos from before the world's birth... they have waited, brooding in the dark places of the world... and now... by digging too deep... we have awakened them.
- --From the Bay12 Games Forums
Forgotten Beasts are subterranean Titans; Forgotten Beasts are, essentially, randomized creatures (or procedurally generated for you fancy big-city developer types) composed from a variety of material types, creature bodies / limbs and other additions. These other additions include everything from venomous stings to flame breath.
The venom of a forgotten beast is randomly generated as are the resulting symptoms. The beast's breath attack is also randomly generated, if it has one. Venom attacks come in a variety of forms, from boiling ichor to trailing dust, and the effects can range from mild pain to complete and instantaneous necrosis or paralysis. Some forms of venom can spread from spatters and contact with your dwarves, eventually infecting your entire fort. Decontaminating your soldiers in shallow running water is one way to deal with this problem.
The number of forgotten beasts cannot be controlled directly, but is influenced by the size of the world. They dwell most often in caverns. Most of them are building destroyers, and are almost entirely immune to traps (They are trapavoid and nostun but can be caged if a web is on the cage trap). Bridges are also less useful, as they cannot be raised or lowered as long as the beast is standing on (or under) them, preventing the traditional magma pit / dwarven atom smasher designs from working. This is probably one of Toady One's ways of making the encounters even more fun.
Most forgotten beasts can be butchered. Some are quite massive and may leave you with hundreds of meat and bone units and dozens of prepared organ units. Some are not.
One may randomly appear when you reclaim a fortress. Also, they may 'ambush' you.
When a forgotten beast appears, the game pauses and you will get a message.
If you need to kill a Forgotten Beast, order your military to move to the location of the beast. Some beasts however are content not to path to your fortress and will stay dormant underground.
- Some Forgotten Beasts whose bodies are made of liquid or gas die or lose limbs on the first hit; in previous versions, these FBs were effectively immortal. Version 0.31.09 changed "made material-based random headless beasts killable" (by combat). That includes randomly generated Titans too.
- Other Forgotten Beasts are extremely difficult to kill due to some body compositions, such as being made of very hard materials. When confronted with such near-invulnerable creatures the only option is usually to use your brain and try to lock it away somehow. Walls stop them. If you can put it in a pit, a clever trapmaker can feed it invaders. If a near-indestructible beast isn't in a position to threaten your dwarves, it can be used to train reluctant marksdwarves.
- One method of defeating nearly combat-invulnerable Forgotten Beasts (those whose bodies are made of rock, for instance) is to cause a Cave-in on top of them. They'll be killed by dropping either natural or constructed walls or floors on them.
- It is also possible to capture them in cage traps if they are webbed, either with a tamed Giant cave spider's web or from a modded creature.