Hi everyone, 

I hope this is the right part of the forum to post.

I'm planning to do a one shot as a DM with the group I play in (so far I've only been a player).

I want to have some fun, and do something special, and so I am planning on doing a horror one shot for Halloween  focused on a shape shifter.

I've decided to split the monster into different versions..

Starts with the weakest, little limbs, making noises, bumping things, attacking (attack style kind of like sturges).

Main monster is an average humanoid type enemy as it begins taking over it's victims and starts getting stronger, it will also have memories of its victims to help it become more intelligent (I'll note on this later).

And the boss version where it has collected so many npc's etc. to become a nightmarish mash of people... and I've already began designing my arctic map.

I was thinking of a way to use save throws or checks to get players/npc's to trust you (or be decieved)

My main issue is balance. I'm having the boss and medium monster version using the Rakshasa creature, tweaking the stats, such as it's vulnerable to fire.., and added some responses.. 

(Like how it would infect someone in combat when facing the boss, and for the other lesser versions of the same monster, how it could scare the characters  [limit on how often it can attempt], and I think it should work.. I defo want it to have shape changer spell, for the medium and boss versions, [which is level 9, but I'm tweaking the spell slots etc.],

Soo what level should my players have? I may have a max of 5 players, I might state that they will need to multi class with a magic class..

Should they start this session as level 14 (standard array)? They are an experienced group of players.

I want it to be tough but not deadly. (I'm hoping it will be a minimum of 1 session but if the players don't finish I don't mind it continuing a couple more sessions).

I'm thinking of creating another character that I give to any players that have been taken over..

(before the actual boss fight I might have it that the medium monster version doesn't kill a character and take its form, but maybe have a save throw or something to allow the player to still be 'alive' within the creature as the player's form.. and after that changes to somebody else, the player taken still has a role to play, such as becoming a ghost and trying to communicate to the other players what's going on..)...

So add the ghost as a sheet or note in the players journal,regarding any checks and saves the player must now use.

(When the monster gets more clever [before the boss fight] and attempts to deceive, and sabotage the other players tying to destroy it, I'm planning on using the whisper part of the chat to ask the player that was killed to persuade other players they 'are not infected'.)

I'm thinking that this monster was thought to be destroyed but instead it evolved whilst dormant due to it having some flashes of memory of its previous encounters.

I'm planning to have magic, psychic damage involved due to its evolution "and to help fit it in the D&D 5e system (which is the only system I've played and feel comfortable with).

I'm definitely excited to attempt this, I just want to make sure it's balanced enough.

Any other thoughts on how it could be improved are defo welcome as well.

Apologies for the long post.

(I'll be allowing PHB and Xanathars too).

xx