The Possession trait of the ghost says:
Possession (Recharge 6). One humanoid that the ghost can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.
So, the ghost has no armor or weapon proficiencies, no Extra Attack, and doesn't get spellcasting. The rules on armor proficiency state:
If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can't cast spells.
So basically the Ghost now does everything at a disadvantage if the target was wearing armor, and is pretty useless offensively anyway.
Last session, I was running Hoard of the Dragon Queen, and my PCs entered room 3E in Castle Naerytar, where I replaced one of the Spectres by a higher CR Ghost. I possessed a Paladin. Am I limited to swinging around a spear, once per turn, with disadvantage, until the Paladin is knocked out?
Best Answer
Your analysis of the relevant rules is correct: the ghost is not proficient with armor or weapons and suffers the consequences. However, the ghost is not, as you say, limited to attacking the rest of the party ineffectively. It can be much worse than that.
Are there any traps or hazards near the party which the ghost is aware of? The ghost can make its victim dash toward these and attempt to trigger them, harming their possessed body and possibly endangering their companions. Are there nearby groups of monsters which would be attracted by a commotion? The ghost can make its victim scream bloody murder as it runs off in their direction. Is the ghost now holding something valuable or useful? Time to throw that down the nearest pit. If nothing else, the ghost has its own body available as a target, and presumably has a much better chance of hurting itself than it would a resisting opponent.
When a ghost possesses a creature, the question it should ask is: what would be the worst thing to do right now? Then it should do that thing.