Guess you guys didn't see Lord of the Rings when the undead come in to save the day vs the then WINNING side. The humans had no chance of victory, but, all of a sudden and how about that JUST LUCKY IN THE NICK OF TIME they appear on these black boats and kill everything in sight. lmao. I bet yall loved that part didn't you?
Actually, I didn't like this.
In the books, the battle at Minas Tirith isn't saved by the undead army. Aragorn, Gimli and Legolas ask for help from the undead (which where mountain people that once swore allegiance to Isildur, but broke their Oath) to help protect the cities in Lebennin (which is an area of Gondor) from the Corsairs of Umbar. Afterwards, the armies of Lebennin where able to use the ships of those Corsairs to travel to Minas Tirith and helped to change the tide of the battle. In the books, the undead never came to Minas Tirith but were relieved from their oath after helping with the Corsairs.
Then at helms deep or was it Minas Tirith when it is about to fall and they ride out to glorious death oh my how about that JUST LUCKY IN THE NICK OF TIME here comes Gandalf with some Lightning Sword of White Light Destruction to destory the then WINNING side along with a horde of horsemen gathered from out of nowhere (did you see him gather them to arms? no you didn't).
It was Helm's Deep... In the books this is explained much better compared to the films. If I remember correctly (and it's been awhile so...) when Éomer fell into disfavour with king Théoden, he left Edoras for the East-mark (which he was in charge off) together with his riders. Because he wasn't in Edoras at the time off Théodens recovery, he didn't ride out to battle with them. After the battle of the Crossing of Isen, when Théoden left for Helm's Deep, Gandalf rode out to find Éomer and his army. So it was with that army they saved the battle at Helm's Deep. That army didn't just materialise out of nowhere...
So, in both these cases there actually isn't a magic army out of nowhere to turn the tide. It might not be explained that well in the movies (which would be hard to do without adding another 15 - 30 minutes to each movie), but it is in the books.
And yes, I AM a bit of a Tolkien fan... 
And Psychoravin, some of your posts really are rather insulting, could you please be a little bit more respectfull to the others here?