The death of King Théoden - The Riders of the Mark arrived after the battle had begun, relieving the forces of Gondor and the Dúnedain. At this time, The Witch-King of Angmar fought with King Théoden, who was consequently slain after falling from his horse.
The death of the Witch-King of Angmar - Seeing her beloved uncle's fall, Éowyn, who had disguised herself as the knight Dernhelm in order to fight, engaged the Witch-King and, with the help of Meriadoc Brandybuck, was able to slay him.
The death of the Steward Denethor - While the battle occurred outside the walls of Minas Tirith, Faramir and Boromir's father Denethor succumbed to the madness of one of the lost palantírs and, sensing his kingdom was about to fall, committed suicide atop a funeral pyre. He attempted to take his wounded son Faramir with him, but Peregrin Took and Gandalf were able to save the captain's life.
The arrival of Aragorn - After retreating into the White Mountains following the battle at Helm's Deep, Aragorn visited the Paths of the Dead and gathered the ghostly army to aid him in rescuing Gondor. They arrived from southern waterways into the Pelennor--the literal "Return of the King."