It clearly states in the Monster Manual on page 279 that the poltergeist is invisible and does NOT have the life drain ability. It has an AC of 12 and 22hp. Listed as a CR2.Yes I know it's going to be tough for the party, but I don't think it will be impossible. It's slam does 3d6 and would likely one shot any one of them, but at the same time, would certainly out itself by attacking in which the other 3 players would probably be able to kill it. I have considered nerfing it's attack damage to 2d6 or 3d4 if I need to. I let the party find a potion of Faerie Fire, but at this point, I don't see them putting 2+2 together to use it on the Poltergeist. They also have a few healing potions so they could survive if a couple of players get KO'd. I think the Poltergeist makes a reasonable BBEG for the party. I do not want to TPK the low level party and will nerf the Poltergeist as much as necessary for them to win. I think as long as the party is trying, I would never kill any of them at level 1. I consider it a practice level.
The problem I'm having, is how to let them figure out what they're up against.
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