I'm trying to set up an ambush in a tower where there are ghosts or some creature like them that has the ability to walk through walls.
If I had a room 20 feet by 20 feet and a party of 5 Medium-sized PCs, both doors secured with them in the room, how many ghosts could I hide in the walls?
This door opens into another room that's enormous, but this room specifically would be magically sealed so that the PCs wouldn't be able to get out until the required number of monsters were defeated. I'm just trying to figure out how many ghosts could be in the walls so that way I could figure out how many to put into each wave.
The party consists of five 6th-level PCs: 1 barbarian, 1 monk, 1 paladin, a cleric and an elemental wizard.
Best Answer
None, or Unlimited
A ghost takes 1d10 force damage every time it ends its turn inside an object. If the ghosts are literally waiting in the walls, they're going to be destroyed in a bit less than a minute.
On the other hand, the ghosts could simply be waiting outside the room. (Or, if you're feeling fancy, they could wait on the ethereal plane.) If you're willing to allow this, then your question becomes "how many creatures can be waiting outside a room?" and of course the answer is unlimited. (They could stand in an arbitrarily long line, for example.)
But your xp budget only goes to four at once
I went to Kobold Fight Club and told it that you had a group of five level-six characters, and I started adding ghosts to the encounter. After the fourth ghost, it told me that the encounter was "deadly" difficulty (8800 adjusted xp, where the "deadly" threshold is 7000xp). If you have more than that at once, you risk killing your adventurers.
Your daily budget is 20000xp, and you could reach that with three groups of three ghosts each.