What published resource first introduced the duergar

In general in play they were ignored or just treated as an abstract language with no further comment.

As to where they came from, here's an answer from Gary Gygax on Dragonsfoot!

As D&D was being quantified and qualified by the publication of the supplemental rules booklets. I decided that Thieves' cant should not be the only secret language. Thus alignment languages come into play, the rational [sic] being they were akin to Hebrew for Jewish and Latin for Roman Catholic persons.

I have since regretted the addition, as the non-cleric user would have only a limited vocabulary, and little cound [sic] be conveyed or understoon [sic] by the use of an alignment language between non-clerical users.

If the DMs would have restricted the use of alignment languages--done mainly because I insisted on that as I should have--then the concept is vaible [sic]. In my view the secret societies of alignment would be pantheonic, known to the clerics of that belief system and special orders of laity only. The ordinary faithful would know only a few words, more or less for recognition.

In other words, it was supposed to be more like religious languages, but wasn't really well thought through. It disappeared in Second Edition and was not missed.

The holy avenger was officially part of the Forgotten Realms since the beginning.

The official beginning of the Realms in a game publishing sense¹ starts in 1987 with the Forgotten Realms Campaign Set—the “Grey Box”—published for first edition AD&D. By that point the holy avenger was already an integral part of AD&D, appearing in the DMG (1979) on page 165.

Note that Supplement 1: Greyhawk is a 1975 supplement for the earlier, original D&D game (dubbed “original D&D” or “0e” (zero-e, oh-e)) published in a box of three stapled booklets in 1974. The holy avenger by any name was a well established part of the game before AD&D or the Forgotten Realms were even conceived of as publishing products at TSR.

The 1987 edition of the Realms predates the idea of AD&D edition changes and the accompanying cataclysms in the Realms. Those were later inventions by minds at TSR after Gygax had been forced out. This makes the holy avenger part of the baseline Realms from which every edition change and cataclysm later derived.

So for the purpose of tracing the origin of the type of magic sword, it’s integral to the Realms, and its AD&D description is the baseline version, before any Realms events that changed how magic works.

1. A note on when official-ness applies to the Forgotten Realms. The origin of the Forgotten Realms predates D&D entirely, circa 1967, as it was an invention of Ed Greenwood’s childhood imagination before he encountered D&D. It’s not possible that anything derived directly from the D&D holy avenger was part of this pre-D&D conception of the Realms. Even when Greenwood was publishing his home game’s details in Dragon articles, it wasn’t “official” at that point even if his home game likely had a holy avenger at least in their rulebooks, as Greenwood’s articles were just a fan sharing his ideas and Dragon at that time often published fan/homebrew material that wasn’t endorsed as “official”. As Greenwood wrote more articles, the line between official and fan eventually blurred, to the point that TSR bought the Realms and published a novel and the Campaign Set in 1987. This answer uses that year as the most meaningful and clear beginning to the “official” Realms. The very first official publication was actually the novel Darkwalker on Moonshae, but only by a month since it was written in tandem with the official campaign set, so that doesn't impact the 1987 start-date.