It is going to be a good while before I actually set this campaign in motion, as the entire concept is a bit of a higher level campaign. Planning it for characters around level 6-9. Now, I could instantly go with undead, ghosts and ghouls, but what fun would that be?
The concept I've currently devised is our group of adventures will find out about this haunted mansion. At first inspection, the mansion seems to be fine, until the party locates a stairwell. Upon the entire party travelling down it, the steps themselves begin to break apart, as though something was hitting them from beneath with an extremely powerful force, cutting off their escape.
To their surprise, although light is down there, the walls constantly flicker with shadows, tons of sounds can be heard, but not exactly pinpointed or determined what exactly it is. Further progression leads them to a long hallway, and as they are traversing through it, writing on the walls suddenly appear, not exactly side by side, but all through out. In addition, if the players had cast any sort of buff spells, they would notice that the effects had not faded yet. (The entire premises of the building is timeless in respect to magic)
As the explorer deeper, more writing, "You are doomed like me. It will get you soon." One room within the underground portion of the mansion has what appears to be a custom summoning circle, with countless modifications to it, although the party may or may not be able to determine this. If, at any point the players completely block off a section of the halls, such as through erecting a barrier, a blood splatter will quickly splash across it, as though someone had just been slashed by a sword or claw, as well as hearing a high pitched, although quick scream.
Finally, they come to the furthest room, which contains a staff laying on the floor. If Identified, they will see that its a slightly obscure Staff of Time Stop, that seems to have a strange side effect of cursing its user to not be able to cast magic as long as the duration of Time Stop occurs, as well as effecting everyone within the vicinity. While spells themselves cannot be cast by those effected by the staff, magical objects will still work. When used, everyone in the room suffers the effects, and it becomes much more apparent what everything else was before. Roars and screams echo, with brief pauses as they traverse more through through the time stopped corridors.
Eventually they will come across a partially sadistic gnome holding a piece of chalk, and telling the players frantically to run, hide, and fear "it". "It" turns out to be a failed experiment of the gnome to create a weapon for one of the nearby lords, and while successful in summoning it, could not be contained and would not even obey its summoner. Now, at this point they could either attempt to kill it (VERY Unlikely... I want the thing to be able to eat the players alive and tear them to shreds in about 2 or 3 hits.), or to help bide time as the gnome attempts to craft a new magical object that can solve the problem in some way (the player can make recommendations as to what to make).
The experimental monstrosity has 4 legs, 2 arms, and can Shadow Jump. It does not have eyes, but does have dark sense. One arm is scythe like, resembling that of a praying mantis, while the other seems morbidly disfigured, with a very thin upper arm, but an inflated lower that has huge claws. In addition to all of these features, it would have fast regeneration while it is in shadows, and is able to create darkness in a small area for 3 rounds.
Overall, I'm attempting to make a scenario where the players can't exactly beat it through brute force, are terrified of what it may be able to do, but is still able to interact with it in various ways, opening up opportunities for creative play. I am, however, lacking in ideas of things leading up to said point. Sure, swarm of rats/bats could be one small part, but not sure what else. Perhaps lesser interments that were failures? Mostly need some additional ideas for fillers leading up this this point.