This item unlocks the hidden appearance called Bonebreaker for the Assassin rogue artifact, The Kingslayers. It randomly drops from any of the rare elite spawns in The Underbelly of new Dalaran.
This item unlocks the hidden appearance called Legionterror for the Destruction warlock artifact, Scepter of Sargeras. It drops from the summoned boss that requires the warlock's Order Hall to have the Tier 2 trait named "Demonic Offering" (The Ritual of Doom).
Unholy Death Knight Hidden Artifact Appearance d00mgiv3r found the Unholy Hidden Artifact earlier tonight, but the trigger to obtaining it isn't yet clear.
Update: Several other players have found the book now. Many of them have mentioned that they have 25+ points in their Artifact weapon and the Reanimation Order Hall talent. They have all been able to get it on a training dummy.
There is a hidden tracking achievement that may be releated: Hidden Tracking - Apocalypse Ghouls Summoned - Number of Ghouls Summoned by Unholy DK Artifact
This item unlocks the appearance of the same name for the Arms warrior artifact, Strom'kar, the Warbreaker. It is the reward of completing the quest Secrets of the Axes. You must find Saurfang in The Broken Isles... and duel him.
This item unlocks the hidden appearance called Crest of the Lightborn for the Holy priest artifact, T'uure, Beacon of the Naaru. It is created by combining two items.
This item unlocks the hidden appearance called Vindicator's Bulwark for the Protection paladin artifact, Truthguard. It is looted from chest at the end of the Withered Army scenario in Shal'aran, Suramar.
To enter the scenario, you need to have completed the main Suramar questline up to Building an Army and have at least 400 x Ancient Mana. (Additional Ancient Mana will allow you to begin the scenario with better odds.)
This item unlocks the hidden appearance called Guardian of the Glade for the Guardian druid artifact, Claws of Ursoc. A player looted it from Ursoc in The Emerald Nightmare (LFR). (The player also noted that the item could be traded, but that sticks out as odd to me, and I would not expect this behavior to be true on live servers.)
This item unlocks the hidden appearance called Watcher's Armament for the Holy paladin artifact, The Silver Hand. It randomly drops from demons in Faronaar, Azsuna.
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Equipping this item also automatically equips the off-hand (technically just another 2-hander), Warswords of the Valarjar.
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This item unlocks the hidden appearance called Bonebreaker for the Assassin rogue artifact, The Kingslayers. It randomly drops from any of the rare elite spawns in The Underbelly of new Dalaran.
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Equipping this item also automatically equips the off-hand, The Kingslayers.
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This item unlocks the hidden appearance called Legionterror for the Destruction warlock artifact, Scepter of Sargeras. It drops from the summoned boss that requires the warlock's Order Hall to have the Tier 2 trait named "Demonic Offering" (The Ritual of Doom).
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This item unlocks the hidden appearance called Bone Reaper for the Unholy death knight artifact, Apocalypse.
From MMO-Champion news page:
Unholy Death Knight Hidden Artifact Appearance
d00mgiv3r found the Unholy Hidden Artifact earlier tonight, but the trigger to obtaining it isn't yet clear.
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This item unlocks the appearance of the same name for the Arms warrior artifact, Strom'kar, the Warbreaker. It is the reward of completing the quest Secrets of the Axes. You must find Saurfang in The Broken Isles... and duel him.
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This item unlocks the hidden appearance called Ravenguard for the Marksmanship hunter artifact, Thas'dorah, Legacy of the Windrunners. It is sold by Veridis Fallon in Azsuna for 1.7K gold (faction price discount included) at Revered reputation with Court of Farondis.
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This item unlocks the hidden appearance called Crest of the Lightborn for the Holy priest artifact, T'uure, Beacon of the Naaru. It is created by combining two items.
Rod of the Ascended is purchasable for 2,000 gold (not sure if faction price discount applies) from Valdemar Stormseeker (Valdisdall, Stormheim) at Exalted reputation with Valarjar.
Crest of the Lightborn drops from Hyrja in Heroic Halls of Valor.
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Players start with a cap of 300 but can increase it in the following ways:
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This item unlocks the hidden appearance called Vindicator's Bulwark for the Protection paladin artifact, Truthguard. It is looted from chest at the end of the Withered Army scenario in Shal'aran, Suramar.
To enter the scenario, you need to have completed the main Suramar questline up to Building an Army and have at least 400 x Ancient Mana. (Additional Ancient Mana will allow you to begin the scenario with better odds.)
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This item unlocks the hidden appearance called Dark Runeblade for the Frost death knight artifact, Blades of the Fallen Prince.
It is a guaranteed drop from The Soultakers world boss encounter in Stormheim.
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Equipping this item also automatically equips the off-hand, Blades of the Fallen Prince.
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This item unlocks the hidden appearance called Guardian of the Glade for the Guardian druid artifact, Claws of Ursoc. A player looted it from Ursoc in The Emerald Nightmare (LFR). (The player also noted that the item could be traded, but that sticks out as odd to me, and I would not expect this behavior to be true on live servers.)
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Equipping this item also automatically equips the off-hand, Claws of Ursoc.
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This item unlocks the hidden appearance called Watcher's Armament for the Holy paladin artifact, The Silver Hand. It randomly drops from demons in Faronaar, Azsuna.