***EDITED WITH SUBTRACTIONS AND ADDITIONS.***
So I started an embark to test ghosts--can they kill goblins? what kinds are there? etc.
The plan is (was) simple--none get burial, none get slabs--everyone remains in Spiralchanted--living or dead.
What I have learned so far:
1. Most ghosts are largely benign--they haunt, lowering happiness, but they do not bother*
*except with doors--they seem to open doors--two farm floods and several hallway-of-freezing-doom fatalities later I can testify that the ghosts even open doors which are mechanically wired closed**
***although, possibly this was all caused by my cave-in traps
Benign type seem to fall into several varieties ( I think I am missing one-
the missing type is the kind that follows one dwarf at a time--I can't find any in the fort right now)
A. Restless Haunts bother relatives and acquaintances, and thus spend most of their time in training rooms, farms, etc.
B. Forlorn Haunts hang near where they died, or another chosen location--e.g. floating in the empty space where the aquifier was frozen (killing several miners) and carved out, but also some random lake in a cavern where no dwarf ever went.
C. Angry ghosts--these will cause problems, killing and maiming. To date I have never observed a ghost injure a goblin or an animal except indirectly via possession**, only dwarves are targeted. Ghosts that possess others never seem to attack directly, just as ghosts that attack directly never seem to possess others, Possessed dwarves are blamed for crimes committed during possession with appropriate criminal consequences (beating, imprisonment etc).
**"Possess" here is not the same as the artifact making possession and as other people's *science* indicates seems to be unrelated. It only refers to when a ghost triggers a tantrum in a dwarf--e.g. "Urist "ghostly bonecarver has possessed Urist Dwarven lumberjack, lumberjack throwing tantrum".
2. Ghosts age--infant ghosts become child ghosts (so far no clue if child ghosts become adult ghosts)
3. Ghosts can acquire coatings--so far sand and water coatings--and only after a cave-in trap went off, I have no experience with magma and can't attempt the trial there.
4. I have not seen a ghost bother anyone other than my dwarves--animals, goblins etc seem to cause no reaction in either party on encountering one another.
5. Child ghosts rarely injure but often break furniture
6. Ghosts seem to be generated by the world-- I have a ghost in my base who was not part of the original embark, he has no profession (listed as dwarf ghost), and he is apparently the creator of an artifact(when checked with 'v'). What this means is frankly beyond me. He later disappeared so
7. A final oddity is the display of ghosts in the "unit" screen. Benevolent "lingerers" are listed in a row. Separately from the lingerers, children ghosts are listed together. So are military ghosts. So are foreign ghosts. In between the listings there are random animals, goblin prisoners etc--what causes the self-categorizing still needs to be determined.
8. It is possible to put goblin ghosts to rest, their names appear in the alt+e engrave slab menu, just at the very bottom of the list. Since I've never seen a goblin ghost despite many, many dead goblins, I have not had a chance to see what this does--perhaps putting them to rest and caving-in the room might stir-up trouble. Need more time for science.
If the citizens of Spiralchants collect anymore useful data about the dwarven beyond I will be sure to post it here.