Void Elves were originally introduced in the previous expansion, Legion, with Alleria Windrunner as their primary point of contact in Battle for Azeroth and Shadowlands.
Void Elves are a sect of High Elves that managed to harness the power of the Void for their own--much like Blood Elves harnessed the power of their Sunwell. Alleria Windrunner, sister of Sylvanas Windrunner, is one of these elves. Now, they've come to join the Alliance, alongside the Light-forged Draenei as two Allied races. Players originally had to complete a reputation grind to unlock Allied Races. Now, with Shadowlands' release, players need only find and complete their introductory quest.
How to Unlock Void Elves Shadowlands
Players will first need to complete the entirety of the Argus storyline in order to get the Void Elf unlocking achievement, "You Are Now Prepared!" The minimum level for this quest is 45, as that is the minimum needed to travel to Argus at all.
The achievement requires players to complete the quests: "The Assault Begins," "The War of Light and Shadow," and "Dark Awakenings." All of this happen on Argus and can be encountered naturally as part of the territory and expansion's story. After this, players should return the Alliance Embassy, where they can pick up the Void Elves' unique recruitment quest.
The quest, itself, is simple. It primarily exists as a walkthrough of the lore behind why these elves decided to join up with the Alliance. Essentially, without spoiling too much of the quest, Alleria Windrunner finds a group of High Elves who were exiled for researching the void as she did. She takes them under her wing, delivers them to the Alliance, and pledges herself to Anduin Wrynn and the faction.
Void Elves are a dark blue recolor of the Blood and High Elf models with darker hair and glowing blue eyes. They are also called the Ren'dorei, similar to the Sin'dorei and Kaldorei.