Umaroth was Vrael's dragon and a cousin of Glaedr. Eragon found his white Eldunarí as well as many others in the Vault of Souls which is found under the Rock of Kuthian on Vroengard. Umaroth acted as spokesman for all of the Eldunarí. He held this position to act as an intermediary between the older, more dreaming dragons, and the younger ones, due to his great experience, but still acting young enough to communicate.
During the Dragon War, Riders Vrael, Oromis, their dragons, Umaroth and Glaedr, as well as a few others, conceived a plan to hide many Eldunarí and dragon eggs within the Vault of Souls to protect them from Galbatorix and those who knew of its location had it wiped from their minds. For a century, the Eldunarí went into a trance to wait until Galbatorix and the Forsworn had left the island. The Eldunarí then reached out with their minds and subtly explored Alagaësia to learn what has changed, eventually learning to manipulate the skeins of tangled energy across the land. Together, they could assist in the formation of the Varden and the downfall of the Forsworn. They even helped Hefring of Furnost in his theft of one of the three dragon eggs Galbatorix had in his possession. Unfortunately, Hefring eventually noticed their mental nudging and became frightened and fled. When Saphria's egg was returned to the Varden, the Eldunarí made preparations in case a new Rider was chosen, informing the werecats about the brightsteel under the Menoa Tree and the Rock of Kuthian. After that, they removed all memory of their conversation from their minds.
When Arya, Saphira's egg's courier, was ambushed by Durza and she was forced to magically transport the egg to Brom, the Eldunarí altered the flow of magic so that it would appear in front of Brom's son, Eragon. As Eragon was slowly developing into a Rider, the Eldunarí then gave him visions of Arya being held captive in Gil'ead. They expected that Brom would be with Eragon and Saphira when they arrived there, but was unaware that Brom had been murdered by the Ra'zac. Nonetheless, they went anyway as they had to find the Varden. The last gift the Eldunarí bestowed upon Eragon was the healing of his back and transformation into an elf-like being during the Blood-Oath Celebration.
Eventually, as Glaedr (now an Eldunarí), Saphira and Eragon, arrived at the Vault of Souls and met Umaroth and the other Eldunarí, who explained all that had happened before assisting them in the final battle against Galbatorix. Umaroth taught Eragon a spell so that he could carry the Eldunarí with him to aid in the battle against Galbatorix.
After Galbatorix's defeat, Umaroth travelled with Eragon and Saphira as they went about lifting Galbatorix's spells and oaths of fealty from the Broddring Empire. Upon meeting with Murtagh and Thorn, Umaroth stated that the Vault Eldunarí were well aware of the hardships they were forced to endure. As Murtagh and Thorn decided to go into a self-imposed exile, Umaroth advised them to avoid certain perilous places in Alagaësia.
Eventually, Eragon returned to Brom's diamond tomb and suggested that the Eldunarí find a way to revive him. Umaroth pointed out that even if they succeeded, they may be unable to restore Brom's mind. As Eragon insisted they try, Valdr, the oldest of the Eldunarí, convinced him to leave Brom to rest.
Umaroth later agreed to Eragon's proposal to travel to the east to help train the new order of Riders and calm the twisted Eldunarí that were previously under Galbatorix's control.
Umaroth was murdered (presumably by Galbatorix and Shruikan) in the Battle at Doru Araeba, but his mind lived on in his Eldunarí.
Umaroth was likely a powerful and large dragon when he had his body. After his death at the hands of Galbatorix his life force went into his Eldunarí. As an Eldunarí, Umaroth retained his vast knowledge and mental power which likely was greater than many Eldunarí and was a vital aid to Eragon during the end of the war.