Ugluk was the captain who commanded the Uruk-hai
Saruman on the march to take Merry and Pippin back. His voice had a
growling sound. He was unusually large and black. He gave credit for
slaying of Boromir to the Uruk-hai, although Pippin had seen that only
of those had been the very large Orcs. He also said they had taken the
and led the other Orcs to their destination and that all should stay
against the horse-riders. If they went to the bridge where the Nazgul
the Uruk-hai would be left on their own against Rohan and Saruman would
see the hobbits, and his own party would be unrewarded. Ugluk did not
how dangerous the Nazgul were and that Sauron would not let them cross
River from the East at that time, although Grishnakh told him.
Ugluk was under orders to bring the hobbits to Saruman alive and unspoiled, while the Orcs of Mordor were under orders to take them to Sauron. The Orcs of Moria wanted to kill the hobbits, take the loot, and go home.
When the time came to stand against the Rohirrim, Ugluk allowed the Northerners to run ahead into the Forest, leaving the hobbits with the Isengarders. The Mordor orcs led by Grishnakh returned for the prisoners and for his own Orcs who had been left behind with Ugluk, too good to lose. The two groups joined and ran for the forest in good order. Grishnakh's men, apparently by his order, let the Isengarders go towards the front gradually. They caught up to the Northerners who had difficulty running in the sunlight. They were already to the outlying trees before the Rohirrim overtook them. The main group of Orcs stayed to fight, although a small band broke away and kept running towards the forest. Grishnakh seemed to allow Ugluk to be the main commander, jeering at his leadership. Ugluk prevented the Orcs from wasting too many arrows shooting towards fires. Ugluk waited for Mauhur and his band who were hidden in the forest before doing a night attack on the riders. This news cheered his fellow Isengarders. Ugluk prevented the Orcs from stampeding when a few riders slipped up in the dark and got off some shots. But when he went to do this, Lugdush and the other two guards went to help him. This left the tied hobbits alone a few seconds, and Grishnakh grabbed them and ran.
Eomer, leader of the Rohirrim attack, dismounted, fought and killed Ugluk.
Reference: The Two Towers: "The Uruk-hai"
"Eomer and Ugluk". Artist Brett Hale. Rolozo Tolkien page