Most ghosts we see are withdrawn or disturbed spirits…
Which after being defeating, they most likely go to rest
YET
Depends on the ghost as well
A troll most likely goes to ol’ Bwonsamdi…
A demon’s or DH’s goes to nether…
And if killed in nether, maybe lays to rest in shadow lands…?
Most likely disturbed ghosts are drawn from their rest in Shadowlands and are disturbed…
Like Sorrow hill ones (graveyard around Uther’s tomb)
The duskwood ghosts -south- of raven hill…
In outland/ draenor, D’ore’s remains drawn draenei spirits to where he was buried… Which later known as a temple known as Auchindoun, and in AU Talador the spirits were disturbed, as Teron’gor and shadow council were causing disturbance…
Then we saw them as ghosts, perhaps because they were disturbed,or maybe their spirits is still in shadow lands but we see it
Although the draenej spirits were able to be summoned, like Yrel’s trial to become ans Exarch, which MAraad, Haturu and some other Draenei spirits were summoned to test her
K’ure also draws orc spirits to remains of Genedar, which later was known as Oshu’gun, mountain of spirits…
In AU Draenor Cho’gall was disturbing them alongside pales, who was also torturing poor K’ure, cause all of these, we saw restless ancestors and spirits, including Golmash’s, grom’s father…
But yeah they are usually angered and disturbed ones, or summoned, maybe tortured …
As we can see in Ashbringer artifact quest in Legion we pray and call upon Uther’s spirit, his spirit appears to us, and guides us…
And perhaps, goes back to his rest…
Ah also we see those of Admiral Taylor’s Garrison, including the Admiral himself who was cursed by Ephial, which later becomes follower and fights…
Perhaps later he was cured, and rests?
Also odyn’s val’kyrs draw worthy vrykul spirits from Shadowlands, ressurecting them as Valarjar… Perhaps drawing them from their rest in Shadowlands…
But as for RP, i belive spirits can be summoned to aid or guide us, which can be cool and epic in dire situations, or simply talking with the ancoestor for blessing, or w/e
or they be hostile who were angered and disturbed, and later are putted to rest…!
And also, perhaps the spirits of those connected to Emerald dream, most likely Druids and Ancients, goes to Emerald Dream… Like Elerethe Renferal after we kill Xavius in Rift of Aln… or Cenarius’, or Ursoc’s…
Ancients’ are known to be ressurected, but dunno about others, like if Elerethe can be ressurected from Emerald Dream or not…
Or even her spirit called upon and communed with… Or even able to be disturbed?
And many other possibilities…