The first Avari that the Eldar met again in Beleriand seemed to have claimed to be Tatyar, who acknowledged their kinship with the Exiles, though there is no record of their using the name Noldo in any recognizable Avarin form. They were actually unfriendly to the Noldor, and jealous of their more exalted kin, whom they accused of arrogance.
This ill-feeling descended in part from the bitterness of the Debate before the March of the Eldar began, and was no doubt later increased by the machinations of Morgoth; but it also throws some light on the temperament of the Noldor in general, and Feanor in particular. Indeed the Teleri on their side asserted that most of the Noldor in Aman itself were in heart Avari, and returned to Middle-Earth when they discovered their mistake; they needed room to quarrel in.
- HoME 11; Quendi and Eldar; The Clan names
It should be noted in this text a lots of Avari was intended to been in Belerieand which AFAIK was rejected later on
The implication that as opposed to Celbin
[S. for all Elves but Avari] the Moerbin
[S. for anyone in ME that do not live in Beleriand] were allies of Morgoth, or at least of dubious loyalty, was, however, untrue with regard to the Avari. No Elf of any kind ever sided with Morgoth of free will, though under torture or the stress of great fear, or deluded by lies, they might obey his commands: but this applied also to Celbin.
The 'Dark-elves', however, often were hostile, and even treacherous, in their dealings with the Sindar and Đoldor; and if they fought, as they did when themselves assailed by the Orcs, they never took any open part in the War on the side of the Celbin. They were, it seems, filled with an inherited bitterness against the Eldar, whom they regarded as deserters of their kin, and in Beleriand this feeling was increased by envy (especially of the Amanyar), and by resentment of their lordliness. The belief of the Celbin that, at the least, they were weaker in resistance to the pressures or lies of Morgoth, if this grievance was concerned, may have been justified; but the only case recorded in the histories is that of Maeglin, the son of E÷l.
The belief of the Celbin that, at the least, they were weaker in resistance to the pressures or lies of Morgoth, if this grievance was concerned, may have been justified; but the only case recorded in the histories is that of Maeglin, the son of E÷l. E÷l was a Mornedhel, and is said to have belonged to the Second Clan (whose representatives among the Eldar were the Đoldor).# He dwelt in East Beleriand not far from the borders of Doriath. He had great smith-craft, especially in the making of swords, in which work he surpassed even the Đoldor of Aman; and many therefore believed that he used the morgul, the black arts taught by Morgoth. The Đoldor themselves had indeed learned much from Morgoth in the days of his captivity in Valinor; but it is more likely that E÷l was acquainted with the Dwarves, for in many places the Avari became closer in friendship with that people than the Amanyar or the Sindar. E÷l found ═rith (34), the sister of King Turgon, astray in the wild near his dwelling, and he took her to wife by force: a very wicked deed in the eyes of the Eldar.
His son Maeglin was later admitted to Gondolin, and given honour as the king's sisterson; but in the end he betrayed Gondolin to Morgoth. Maeglin was indeed an Elf of evil temper and dark mind, and he had a lust and grudge of his own to satisfy; but even so he did what he did only after torment and under a cloud of fear.
Some of the Nandor, who were allowed to be Celbin, were not any better. Saeros, a counsellor of King Thingol, who belonged to a small clan of Nandor living in eastern Doriath, was chiefly responsible for the driving into outlawry of T˙rin son of H˙rin. T˙rin's mother was named Morwen 'dark maiden', because of her dark hair, and it was one of Saeros' worst insults to call her Morben. For that T˙rin smote him in the king's hall.
’This resentment on the part of the Avari is illustrated by the history of PQ *kwendi. This word, as has been shown, did not survive in the Telerin languages of Middle-earth, and was almost forgotten even in the Telerin of Aman. But the Loremasters of later days, when more friendly relations had been established with Avari of various kinds in Eriador and the Vale of Anduin, record that it was frequently to be found in Avarin dialects. These were numerous, and often as widely sundered from one another as they were from the Eldarin forms of Elvish speech; but wherever the descendants of *kwendi were found, they meant not ’Elves in general’, but were the names that the Avari gave to themselves.
... The form penni is cited as coming from the ’Wood-elven’ speech of the Vale of Anduin, and these Elves were among the most friendly to the fugitives from Beleriand, and held themselves akin to the remnants of the Sindar.
- HoME 11; Quendi and Eldar; Author's notes to Quendi and Eldar; Note 9