Uldor

After entering Beleriand, Uldor along with his father and his brothers joined the service of Caranthir, and settled in Thargelion.

Ultimately though, his father Ulfang's purpose from the beginning was to eventually betray the Elves, which his sons ultimately did in the Battle of Unnumbered Tears, under Uldor's leadership - they turned against their allies, attacking them in the rear. Their betrayal proved crucial in the fight against Morgoth's forces, though, and Morgoth was victorious.

However, Uldor's brothers Ulfast and Ulwarth were killed by the sons of Bór, who remained loyal to the sons of Fëanor during the battle - while Uldor himself was slain by Maglor.

The name Uldor was probably not the character's true name, as it seems to be Sindarin for "Ugly lord": ul- ("ugly") + dor ("king, lord").

A curious thing about the name Uldor is that it remained unchanged from its conception to the later legendarium.

Earlier versions of the Nirnaeth

According to earlier versions of the legendarium, instead of being killed by Maglor in the Battle of Unnumbered Tears, Uldor was slain by Cranthir (an earlier name for Caranthir) instead.

It is only in The Grey Annals (c. 1951) that the story of Uldor's death at Maglor's hand appears, as well as the idea that Uldor summoned more Men from east of Beleriand.

Uldor's year of birth

In the appendix of The Lost Road and Other Writings in The Genealogies (dating from the early 1930s), it is stated that Uldor was born in the year 125.

When comparing the above date with its associated text The Earliest Annals of Beleriand, Uldor was born 38 years before the Swarthy Men entered Beleriand in the year 163.

Also, the dates found in The Earliest Annals of Beleriand were by and large moved 300 years into the future in The Grey Annals (c. early 1950s), such that, for example, an event happening in the year 195 in The Earliest Annals now happened in the year First Age 495.

Later legendarium

In the Narn i Chîn Húrin, an entirely new conception of the Battle of Unnumbered Tears (Nirnaeth Arnoediad) emerged.

According to that conception, the eastern army of Maedhros (called Maedros in this text) never rendezvoused with the western army of Fingon and Turgon at all - in other words, they were not simply delayed due to the actions of Uldor, but were completely cut off from the western army by a large host of Orcs and were subsequently routed.

And while the idea of the treachery of the Easterlings was retained, it was only mentioned in passing.