If you don't have already seen the trilogy, maybe it's better for you to don't participe to this branch, some spoil may appear.

In "The Return of the King", during the battle of Minas Tirith we see the Army of the Dead. As they are dead, no one can kill them, right ?

So my question is quite simple.

Only one man would have been sufficient to win this battle as he is immortal, no ?

#wtf

It's been a while since I've read the books, but am I right in understanding that the main weapon of the 'Dead Men of Dunharrow' was fear? I think the advantage they gave was more getting rid of the Corsairs rather than anything else.

I understand and I agree your answer. But my question wasn't it. I was just wondering about the fact that one dead man would have been sufficient to kill everybody on The Middle Earth ?
Because they aren't only using fear, they can kill and nobody can kill them.
(I'm talking about the movies, because I didn't read the books).

Well, if they all just killed everyone, it wouldn't have been much of a movie then, would it? I think a lot of that stuff was just artistic licence and it's much easier to get away with plot holes in a movie.

They were only bound by their oaths to help out in the particular situation they were asked so they couldn't really rampage across Middle Earth.