Xal'atath, Blade of the Black Empire

Xal'atath, Blade of the Black Empire is one of the artifacts used by shadow priests in World of Warcraft: Legion. Equipping it will automatically place Secrets of the Void in your off-hand.

Source
This item is a quest reward from Blade in Twilight.

Base
Hidden trait: Xal'atath will occasionally whisper to you, similar to other Old God-themed weapons.

Quotes
When equipped, certain zones, characters, and events can trigger Xal'atath to say or whispers things to the player, like the Skull of the Man'ari.

Killing a target

 * whisper
 * whisper
 * whisper
 * whisper
 * whisper
 * whisper
 * whisper

Characters

 * whisper (at Cordana Felsong)
 * whisper (at Glazer)
 * whisper (at God-King Skovald)
 * whisper (at Odyn, in the Seat of Ascension, note that this triggers a fatal interaction)
 * whisper (at Tirathon Saltheril)
 * whisper (at Illysanna Ravencrest)
 * whisper (at Lord Kur'talos Ravencrest and Latosius)
 * whisper (at Arch-Druid Glaidalis)
 * whisper (at Shade of Xavius)
 * whisper (at Ularogg Cragshaper)
 * whisper (at Dargrul)
 * whisper (at Millificent Manastorm)
 * whisper (at Mindflayer Kaahrj)
 * whisper (at Blood-Princess Thal'ena)
 * whisper (at Fel Lord Betrug)
 * whisper (at Lady Hatecoil)
 * whisper (at Wrath of Azshara)
 * whisper (at Harbaron)
 * whisper (at Helya in the Maw of Souls)
 * whisper (at Nythendra)
 * whisper (at Nythendra and Dresaron)
 * whisper (at Il'gynoth the Heart of Corruption)
 * whisper (at Elerethe Renferal)
 * whisper (at Ursoc)
 * whisper (at the Dragons of Nightmare)
 * whisper (at Cenarius)
 * whisper (when facing Xavius)
 * whisper (interacting with the Remnant of the Void after defeating Xavius)
 * whisper (at Ivanyr)
 * whisper (at General Xakal)
 * whisper (at Advisor Vandros)
 * whisper (at Skorpyron)
 * whisper (at Star Augur Etraeus)
 * whisper (at Krosus)
 * whisper (at Spellblade Aluriel)
 * whisper (at Star Augur Etraeus)
 * whisper (at Elisande)
 * whisper (at )
 * whisper (at Thrashbite the Scornful)
 * whisper (at Mephistroth)
 * whisper (at Goroth)
 * whisper (at Mistress Sassz'ine)
 * whisper (at Sisters of the Moon)
 * whisper (Sisters of the Moon defeated, note that this triggers a fatal interaction)
 * whisper (fatal interaction when attempting to absorb Elune's power)
 * whisper (at The Desolate Host)
 * whisper (at Maiden of Vigilance)
 * whisper (before the Fallen Avatar)
 * whisper (at Kil'jaeden)
 * whisper (Kil'jaeden defeated)
 * whisper (at Talixae Flamewreath)
 * whisper (before facing Arkhaan)
 * whisper (at Locus-Walker)
 * whisper (to Alleria at the Seat of the Triumvirate)
 * whisper (at Viceroy Nezhar)
 * whisper (at L'ura)
 * whisper (after L'ura's defeat, when Alleria absorbs her power)
 * whisper (entering the Temple of Anguish, referring to the shivarra)
 * whisper (at Varimathras)
 * whisper (at Dark Keeper Aedis)

Locations

 * whisper (in the Broken Isles)
 * whisper (in the Broken Isles)
 * whisper (in the Broken Isles)
 * whisper (in the Hall of Glory in the Halls of Valor)
 * whisper (in the Maw of Souls)
 * whisper (in Netherlight Temple)
 * whisper (in Netherlight Temple)
 * whisper (in Netherlight Temple)
 * whisper (in Netherlight Temple at the Sanctuary of the Void)
 * whisper (in Netherlight Temple)
 * whisper (in Helheim)
 * whisper (in Orgrimmar)
 * whisper (at Blackrock Mountain)
 * whisper (in Sunwell Plateau)

Azsuna

 * whisper
 * whisper
 * whisper
 * whisper

Highmountain

 * whisper
 * whisper
 * whisper

Val'sharah

 * whisper
 * whisper
 * whisper
 * whisper

Stormheim

 * whisper
 * whisper (in Hrydshal)

Suramar

 * whisper
 * whisper
 * whisper

Broken Shore

 * whisper
 * whisper
 * whisper
 * whisper
 * whisper
 * whisper
 * whisper (at Deliverance Point)

Argus

 * whisper (aboard the Vindicaar)
 * whisper (at Mac'Aree)

Antorus, the Burning Throne

 * whisper (at Light's Breach in Antorus, the Burning Throne)
 * whisper (in the Hallowed Felworks, with all the fel reavers)
 * whisper (at the Seat of the Pantheon)

Battle for Azeroth

 * whisper (used in a blue post by Kaivax to announce the Crucible of Storms release date)
 * whisper (at the Locus of Infinite Truths in Ny'alotha, where N'Zoth is located)

Other

 * whisper (after beating all raids in a single expansion)

Unused

 * whisper
 * whisper (possibly in reference to The Soultakers in Stormheim, as one of the bosses is Reaver Jdorn)
 * whisper

Trivia

 * Its flavor text was changed from "Do not put your faith in the black blade. It will deceive you. It will not be controlled." during the Alpha.
 * For the main skins, the cutting edge of the blade faces towards the wielder, while a pointed talon section is used against enemies.

Speculation
Kaivax's use of the quote "whisper" in relation to the Crucible of Storms under the Shrine of the Storm, which is part of N'Zoth's body, in combination with the Ny'alotha quote is "whisper" and her mentioning in the quest Twist the Knife that the Shrine of the Storm is a place of ancient power that has had many names over the ages, might indicate that the Shrine of the Storm is where she had been used previously.