As promised, here's a rundown of the PCs in my Eberron game. (Note that while I'd like nightmares that are relevant to their characters, I'm more concerned that they should be unsettling to the players.)
1. Captain Fenris. Human male ranger/cleric. Born to an aristocratic Cyran family, enrolled in the army, fought in the Last War. Hatred of undead and Karrnath's use of them. Fighting against Karrns at the border on the Day of Mourning, fled the mists. Two childhood friends enrolled with him, both now dead, one (his lost love Siobhan) sired as a vampire by the Blood of Vol. She retained hatred of Blood of Vol, dead again, Fenris still mourns her. Seeks to join the Red Watchers (undead hunters who oppose Vol).
2. Chanterel. Changeling female warlock. Enslaved by the Emerald Claw, freed by the rogue vampire Siobhan. Now wears Siobhan's face (as a vampire) in memory of her deceased rescuer. Hasn't come to terms with the fiendish source of her warlock power.
3. Ardente d'Lyrandar. Male half-elven bard. Dragonmarked, favoured in house. Works on airships, dreams of captaining one. Friends with Harlyn dÓrien.
4. Harlyn d'Orien. Human male warmage. Dragonmarked, but relations with House Orien are a little strained. Enlisted with the Aundairian forces in the Last War, set to guarding a research facility. Released to House duties on Orien's request. Subsequently learned that the facility was destroyed, and his company missing, presumed dead. Self-described poncy character. Frequently starts encounters by either offering or seeking fine liquor.
5. Seraphina Jorasco, halfling female healer. Favoured in House, not yet dragonmarked. (The player plans a Siberys mark.) Spent time in the Eldeen Reaches learning herbal rememdies from the druids there. Developed a hatred for aberrations from the Gatekeepers.
6. Sil, changeling female rogue. Likes the 'thousand faces'lifestyle. Native of Sharn, did some freelance work for Breland during the Last War, engaged in less legal activities on her own behalf too. The player is interested in a future with either the Brelish intelligence or the Cabinet of Faces. (Or both.)
7. Xathris, male shifter ranger. From the Eldeen Reaches. Not much character detail yet, some speculation that he has a conjoined twin. (The player is resistant to the idea, everyone else quite likes it.)
The main thing I want the nightmares to accomplish is to unsettle the players themselves. Secondary goal is to be relevant to their PCs too. (Obviously some PCs have stronger hooks than others.) The final goal is that the details of some of the nightmares may prove to be premonitions of one sort or another.
On that note, for Seraphina I'm thinking an Equus-style Aztec sacrifice nightmare, with the sacrificial knife being passed to her by the party's patron, a Morgrave University professor specialising in the history of the Dragonmarked houses, and secretly a silver dragon of the Chamber. (They're in the Mournland on his behalf.)
For one of the others, probably either Harlyn or Fenris, I picture them back at their home village to be greeted as a returning hero, with lots of handshaking and backslapping and encouragements to "hurry up, they're waiting for you!" Repeats as they approach the town hall, but if they turn around, they find the villagers they've already freeted now follow them in a shuffling, lurching gait with sunken, hollow eyes. When they enter the town hall, they find their parents, heads bowed, backs turned, rasping, near-inhuman breath, and chains hanging from loose shackles on their wrists. Behind them, the dead-eyed villagers crowd in, blocking their exit and pressing them forward. Their parents straighten and begin to turn around...
So that's it. Oh, and some stuff about cigars and a tunnel. Any ideas?