Standard ghosts in Pathfinder don't have a means to disappear, since they lost their connection to the ethereal plane/manifest ability from 3.5e.
Would it be reasonable to give a ghost natural invisibility as one of its ghostly special attacks?
I'd want to give it the ability to switch between visible and invisible at-will as well, possibly as a swift action (so it can do it 1/round, but doesn't lose an attack or move action). In that case I'm not sure if the invisibility could be natural invisibility though, since it would have a visible form.
Invisibility as a constant spell-like ability is also an option, but that would be susceptible to Invisibility Purge, I think.
What do you guys think?
Having something be incorporeal and invisible makes an opponent incredibly difficult to combat. You have 50% miss chance from invisibility (unless they can negate it) and take 50% damage unless they have ghost touch weapons.
All in all this is recipe for seriously hurting a party unless they are prepared for this type of encounter.
I didn't think it would be that severe of a change because the examples of creatures with both begin occurring at pretty low level:
poltergeist is incorporeal and has natural invisibility and is CR 2.
Lares (from the Bestiary in "What Lies in Dust") are invisible and incorporeal and they're CR 5.
Shadow demons are incorporeal and their shadow blend ability makes them functionally invisible in anything short of bright light. They're CR 7.
Admittedly, poltergeists and lares have no direct melee attacks. They attack by telekinesis or possessing objects, but shadow demons, which have sunlight powerlessness, are mostly going to be encountered in conditions without bright light.
I'd really like to run my ghost more like a shadow demon, with it using malevolence to possess people while it's invisible. Otherwise it's pretty obvious that somebody just got possessed when the ghost disappears into them.
The shadows in my campaign have a lesser form of shadow blend too, and are invisible in conditions less than normal light, so my players already do have some exposure to invisible incorporeal creatures. (There's also a lar in the campaign, but it is allied with the party.)
I suppose one solution would be to tie the invisibility to the malevolence ability, like it's the only effect that functions while the ghost is invisible, but it has to be visible to actually attack with corrupting touch.
Or I could give it corrupting gaze, which is a gaze attack, and would only function while it isn't invisible. So the party gets a benefit by it being invisible too (no gaze attack).
The CR of the creatures account for the fact that they have both of those abilities with other weakness to make it fair.
As you note, the poltergeist doesn't have a normal attack method. It can freighten or use telekinesis. Telekinesis from them is fairly weak as they can only use it at caster level 3 and have a low bonus to hit with the ranged attack rolls (+3).
Shadow Demons have a big weakness to light, a spell which isn't unpopular with adventurers. Also, they have to use a move action to become invisible, which is broken as soon as they attack. Which by the way will limit them to only a single attack because they've used their move action. And unless adjacent to an enemy already will need to turn invsible as a move and move toward you.
So, I get that you're saying at-will invisibility wouldn't be a good fit for a swap on a ghost special attack because it is too powerful. So strike that idea.
What kind of CR adjustment would a ghost that does have natural invisibility or at-will invisibility be looking at?
Standard ghost template is CR +2. So if the ghost IS also invisible, what kind of CR should it have?
Not to mention you would literally always get a suprise round on the party if they are not running around with true seeing everywhere. I mean, I really doubt you can make a perception check vs something that does not disturb the air or anything, is walk-through, makes no sounds when moving, AND is invisible.
What about a ghost that can stay invisible indefinitely but can't attack while invisible except to use its malevolence (possession) ability?
Would Glitter Dust work on said Ghost?
Not to mention you would literally always get a suprise round on the party if they are not running around with true seeing everywhere. I mean, I really doubt you can make a perception check vs something that does not disturb the air or anything, is walk-through, makes no sounds when moving, AND is invisible.
You still get a perception check. With that aside a ghost that goes invisible as a swift action is a tough opponent. To answer the other person the glitterdust has a 50% chance of effecting the ghost.
What about a ghost that can stay invisible indefinitely but can't attack while invisible except to use its malevolence (possession) ability?
That is still really brutal. I would not use the ghost against a party that was not able to deal with it in some way. That does not mean the party has to have see invis or another method to deal with invisible creatures prepped that day, but they would need a way to gain access to it, even if the ghost runs them off the first time.
I'm really looking to mimic the D&D 3.5e ghost's ability to Manifest that wasn't carried over into Pathfinder. Basically, in 3.5e ghosts were invisible except when Manifesting because they normally dwelt on the Ethereal Plane and only "manifested" on the Material Plane. The ability to move back and forth between the Material plane and the Ethereal Plane was removed from the Pathfinder ghost template.
Here's the 3.5e text for Manifestation, a property all ghosts had:
A manifested ghost can strike with its touch attack or with a ghost touch weapon (see Ghostly Equipment, below). A manifested ghost remains partially on the Ethereal Plane, where it is not incorporeal. A manifested ghost can be attacked by opponents on either the Material Plane or the Ethereal Plane. The ghost's incorporeality helps protect it from foes on the Material Plane, but not from foes on the Ethereal Plane.
When a spellcasting ghost is not manifested and is on the Ethereal Plane, its spells cannot affect targets on the Material Plane, but they work normally against ethereal targets. When a spellcasting ghost manifests, its spells continue to affect ethereal targets and can affect targets on the Materal Plane normally unless the spells rely on touch. A manifested ghost's touch spells don't work on nonethereal targets.
A ghost has two home planes, the Material Plane and the Ethereal Plane. It is not considered extraplanar when on either of these planes.
I see in rereading the 3.5e Malevolence ability that only a manifesting ghost can possess someone though, so I guess it's a moot point. It does make it hard for someone to secretly become possessed.
I suppose the RP for the ghost going into someone could be that the ghost vanishes.
I still like the idea of invisible, incorporeal ghosts haunting a site with most people none-the-wiser. I suppose poltergeist sort-of fills that niche though.
OK, I think I've got this figured.
The ghost could have invisibility at-will as a swift or immediate action, but can't attack while invisible, only attempt possession, and takes a full-round action to appear?
That would negate surprise rounds from that form of invisibility and would also discourage its use in combat.
I don't know though, I feel funny tampering with how coming out of invisibility works.