Fish Fridays feature: The Darkscale Naga, the Nazj'vel digsite, and the two battles for Darkshore
(The Darkscale appear in two battlefronts in Darkshore: one is the battle against the Twilights' hammer "Masters' Glaive" cult during the Cataclysm expansion, and the other is the Alliances' battle versus the Horde for control of Darkshore in Battle for Azeroth expansion. I will be covering both here.)
The Darkscale naga first appear at the site of a Night Elven archaeological digsite near the Twilight Shore in Darkshore, the Nazj'vel digsite. In World of Warcraft: Cataclysm, the hero is sent to slay the naga guarding the digsite and obtain the "Horn of the Ancients", an ancient artifact the naga are trying to give to their queen, Azshara. The questline starts with Malfurion Stormrage giving you the "Mounting the Offensive" quest and ends with the druid Balren of the Claw giving you the quest "Ashes in Ashenvale". If the hero slays all four Darkscale naga Priestesses surrounding the Horn of the Ancients, Queen Azshara herself will appear.
A Hearthstone illustration depicting the "Horn of the Ancients", an artifact Malfurion Stormrage and the hero use to defeat the Avatar of Soggoth in the battle for Darkshore in the Cataclysm expansion. (image source: Wowpedia).
The questline culminates in the killing of the Avatar of Soggoth and, having thwarted the Darkscale nagas' schemes, the hero is sent to deal with Garrosh Hellscreams' invasion into Ashenvale.
During the Battle for Azeroth expansion, however, the Darkscale re-appear. During the climactic battle for Darkshore between the Horde and the Alliance, the Darkscale themselves invade the Twilight Shore, ending up in Ameth'Aran and taking prisoners in both sides of the Alliance/Horde conflict. Finally, led by the Lady Janira, they overtake the Ruins of Lornesta, depending on which faction takes control of the area--when the Horde take control, the Darkscale keep the ruins, but when the Alliance takes control, the Allied soldiers keep the ruins.
The Darkscale naga hold the ruins of Lornesta. This was not always the case during Battle for Azeroth--the Alliance sometimes take control, ousting the Naga from the area. (Source: Wowpedia)
It should be noted that around the time of Battle for Azeroth a cave opens up in the area containing Azerite, namely the Lornesta Mine. It would stand to reason, therefore, that the naga were looking to farm the precious metal for their own nefarious ends.