Over the weekend I ran an encounter with several infernal ghosts (RoP:I pg 36). The ghosts are the "spawn" of a demon, whose victims killed their families (and themselves) with a power similar to "Burning Curse" (RoP:I, pg 76). When the ghosts return, they haunt other families, and pass on the curse. They also have a power to throw little burning coals in combat. I gave the ghosts Infernal Might.
My theory here is that their essential nature is that of a ghost, but their powers are of an infernal nature.
One of the players is a necromancer, and prior to the encounter was able to erect a ring of warding against Spirits (ReMe). So as the encounter progressed into combat, the ghosts were stopped by the ward (due to their ghostly essential nature). The coals, however, were able to pass through the ward, being infernal bits of fire. The ghosts were able to be destroyed by "Lay to Rest the Haunting Spirit." (PeMe)
Afterwards, the necromancer player was confused, because he felt that either A) the coals should also have been stopped by the ward, or B) "Lay to Rest" should have been ineffective because they are of an infernal nature.
Did I mess this up? I don't think so, but it makes me wonder.
"Lay to Rest..." (AM5 pg 150) doesn't say anything about the Might having to be magic-realm-affiliated.
Most demons have a second form that they are susceptible to (if penetration). A demon wolf can be targeted by Animal spells, for example, or a demon in human form can have its physical body injured by a PeCo. There's no reason why Mentem spells can't affect ghosts.
The only wrinkle is that both ArM and RoP:I imply that wards must be realm-specific; that is, Ring of Warding Against Spirits is potentially not the same ReMe (General) spell as the ReMe (General) Ward Against Ghostly Demons (see RoP:I pg. 33 for similar infernal-specific wards for ReAn and ReAq); the former is for magic-aligned ghosts. That said, one could find that reading an obstacle to fun, and read the rules more liberally. There's no similar restriction for the Perdo spells.
If the infernal ghost is throwing coals, as a non-Hermetic power the entity could be creating a real item without expenditure of vis, so the moment it leaves the ghost's "hand" the coal is a tangible mundane item moving as if hurled by the Invisible Sling of Vilano, that is, it can only be warded against by an Ignem ward and will not be stopped by any other ward, a PeVi spell, or Parma Magica. Magi should still be able to soak based on their Ignem score, though.
Yeah, I think the official line is that wards have to be realm-specific. (so a Mentem ward vs. ghosts has to be a MAGIC ward against ghosts, or a FARIE ward vs ghosts or whatever.) However, that makes form-specific wards even more useless than they already are. I personally rule that 'sometime in the past, an integration has occurred such that form-specific wards do not need to be realm-specific - thus Ignem wards affect ALL Ignem-aligned spirits (regardless of realm), Ghost wards affect ALL ghosts (regardless of realm), etc."
Though in the LoH p.27 Tome of Binro there is an experimental spell Ward Against Woodland Spirits "against all creatures with a Herbam-based Might Score, irrespective of their supernatural realm, of 10 or less". Maybe IYC Binro was more successful and appreciated than in canon Mythic Europe?