The sound of war was about to fill the fields of the Pelennor. Indeed, war-horns and heavy footsteps sounded all over the place, and the ships of Aragorn and his company were not there yet. But indeed, the Rohirrim had answered the call of Gondor, and the Riders of Rohan had entered the Pelennor fields. The voice of Théoden King of the Mark, suddenly sounded in the field of onslaught:
Arise, arise, Riders of Théoden! Fell deeds awake; fire and slaughter! Spear shall be shaken, shield be splintered, a sword-day, a red day, ere the sun rises! Ride now, ride to Gondor!, said Théoden.
With that, he seized a great horn from Guthlaf, his banner-bearer, and he blew such a blast upon it that it burst asunder. And immediately, all the horns in the host were lifted up in music, and the blowing of the horns of Rohan in that time was like a storm upon the field, and a thunder of the mountains.
And thus the battle started. The forces of the Dark Lord now could not assault the city, if they did not destroy the Rohirrim force first. Many deeds were done during the battle, though I cannot say I know all of them. A significant mention, though, is the slaughtering of the Southron chieftain, the Black Serpent, by King Théoden himself.
But it was no brigand that led the assault upon Gondor. The Witch-King and Gothmog, the Dread Terror, soon came, and darkness filled the plains. Orcs and fell-men started marching upon Minas Tirith. The fight went on, with heavy losses on both sides. But then, the Witch-King and his fell-beast landed in front of Théoden, wounding him fatally. Éowyn was nearby, and she challenged the Witch-King to a fight.
The Witch-King immediately answered with his cold voice: Come not between the Nazgûl and his prey! Or he will slay thee in thy turn., said the Nazgûl.
And Éowyn replied: Do what you will, but I will hinder it, if I may.
Hinder me? Thou fool. No living man may hinder me!, replied the Witch-King.
And Éowyn replied one last time: "But no living man am I! You look upon a woman. Éowyn I am, Éomund's daughter. You stand between me and my lord and kin. Begone, if you be not deathless! For living or dark undead, I will smite you, if you touch him."
And after that, she smote him with her sword, and he made a sound more horrible than all the evil deeds of Arda combined, and he died. Then, they, and some more Rohirrim that were nearby, were mourning the fallen king, but Éomer told them not to mourn yet; open war was in their front. Éomer's rage was immense; thinking his only sister was dead, and he went on shouting ''Death! Death! Death take us all!" all over the battlefield.
At the same time, Faramir, son of Denethor, Steward of Gondor, was also gravely wounded. Despairing at the visions of defeat that Sauron had sent him via his palantír, and believing Faramir to be beyond aid, Denethor prepared to burn himself and his son upon a funeral pyre. Only the intervention of Peregrin Took and Gandalf saved Faramir, but Denethor burned himself alive before they could prevent him.