DM'ing a game where one of my players wanted to play a custom warlock with a demonic arm. I came up with a subclass that would fit the backstory, which was that the character went to hell and a demon severed its own arm in a ritual to send the character back to the real of the living to complete a task for it. I wanted to use the demonic arm to help me sometimes drive the story forward and so it has a mouth on it that the demon can speak through at times. It has served me well but now we are nearing the end of the campaign's story arc, and I don't know how to conclude it and get everything that I want.
Without adding too much unnecessary context, the demon wants to escape hell and has tasked the warlock with finding an item. What I have in mind is that the moment the players get to the room with the item (a magic candle), the demon will take control of the arm, light the candle and grab it, using it to open a portal to itself and come through.
I then want to have some sort of a battle between the warlock and the demon for control of the arm. I ultimately want the player to win this battle, keep the arm, and the demon teleport away only to come back later in the campaign.
What I'm struggling with is how to do this battle over control of the arm. I am thinking that maybe the warlock and demon will battle over control of it in their minds (a separate battle) while the rest of the players can then choose to attack the demon physically while it's... sort of... incapacitated form the "mind battle"? I'm struggling to find a good way for this to resolve climatically...