Gothmog General of the Armies of the Pelennor
“Gothmog the lieutenant of Morgul had flung them into the fray;
Easterlings with axes, and Variags of Khand. Southrons in scarlet, and out of Far Harad black men like half-trolls
with white eyes and red tongues.”
LOTR:ROTK B5 C6
Little is known about the one they call Gothmog, second Lieutenant to the Witch-king of Angmar, who led the forces from Minas Morgul against the city of Minas Tirith. He is only mentioned in passing in the Red Book of Westmarch, as the one who led the armies of Mordor after the fall of the Lord of the Nazgûl on the bloody fields of the Pelennor. A second force lay hidden in the the ruins of Osgiliath and when the first army faltered under the charge of the Rohirrim, they were unleashed by Gothmog and sent streaming onto the field of Battle. They would have overrun the Horse Lords of Rohan, if not for the Black Ships manned by Aragorn that came up out of the south.
This second army had been held in reserve to sack the city after the gate was broken, however they were used to try and stem the time of the Rohirrim and the Black Ships from the south, which has some how turned against them.
There are other tales told of Gothmog, who was said to have been a powerful Uruk, deformed as may of the orcs of Morgul were, by the drinking the fetid and unclean waters that flowed from out of the Dead City. Cunning and intelligent, he had risen up among the ranks of Orc Captains. to be the second Lieutenant to the Witch-king of Angmar, who led the forces of Minas Morgul in the first major assault in the War of the Ring.
He was said to have been killed in Battle by Aragorn, near the slain ruins of the Fell Beast, which saved the life of Éowyn, who had been wounded by killing of the Lord of the Nazgûl.
And now the fighting waxed furious on the fields of the Pelennor; and the din of arms rose upon high, with the crying of men and the neighing of horses. Horns were blown and trumpets were braying, and the mûmakil were bellowing as they were goaded to war. Under the south walls of the City the footmen of Gondor now drove against the legions of Morgul that were still gathered there in strength. But the horsemen rode eastward to the succour of Éomer: Húrin the Tall Warden of the Keys, and the Lord of Lossarnach, and Hirluin of the Green Hills, and Prince Imrahil the fair with his knights all about him.
Not too soon came their aid to the Rohirrim; for fortune had turned against Éomer, and his fury had betrayed him. The great wrath of his onset had utterly overthrown the front of his enemies, and great wedges of his Riders had passed clear through the ranks of the Southrons, discomfiting their horsemen and riding their footmen to ruin. But wherever the mûmakil came there the horses would not go, but blenched and swerved away; and the great monsters were unfought, and stood like towers of defence, and the Haradrim rallied about them. And if the Rohirrim at their onset were thrice outnumbered by the Haradrim alone, soon their case became worse; for new strength came now streaming to the field out of Osgiliath.
There they had been mustered for the sack of the City and the rape of Gondor, waiting on the call of their Captain. He now was destroyed; but Gothmog the lieutenant of Morgul had flung them into the fray; Easterlings with axes, and Variags of Khand. Southrons in scarlet, and out of Far Harad black men like half-trolls with white eyes and red tongues. Some now hastened up behind the Rohirrim, others held westward to hold off the forces of Gondor and prevent their joining with Rohan.
It was even as the day thus began to turn against Gondor and their hope wavered that a new cry went up in the City, it being then midmorning, and a great wind blowing, and the rain flying north, and the sun shining. In that clear air watchmen on the walls saw afar a new sight of fear, and their last hope left them.
From The Lord of the Rings: The Return of the King in the Chapter ‘The Battle of the Pelennor Fields’