Antorus, the Burning Throne: The Death of a Titan

This is completable on LFR :)

[item=152626] is a new legendary added in Patch 7.3.2. Below is a comprehensive summary of how this ring works.
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DISCLAIMER: This information is from the PTR, and some of it may be subject to change on Live servers.

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[item=152626] is the only legendary available as a quest reward. It is granted on completion of the quest Antorus, the Burning Throne: The Death of a Titan, which requires that you kill [npc=124828], the final boss of the [zone=8638] raid. As such, it won't be available until the raid opens.

It's effect is to increase the effect of Light and Shadow powers granted by the Netherlight Crucible by 50%. These are tier 2 powers associated with the 3 socketed relics in your Artifact Weapon. See the Netherlight Crucible section of the Legion Artifact Weapon guide for more details on the crucible.

A few further details on the ring:

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• This legendary is Unique-Equipped: Legion Legendary (2); as such, it respects the 2 legendary limit and cannot be equipped if you have two other legendaries equipped.
  
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  • In contrast, the other legendary added in 7.3.2, [item=154172], DOES NOT share an equip limit with other legendaries - it can be equipped in addition to 2 other legendaries.
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• As this legendary is available after the release of [zone=8638], it will be item level 1000.
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Below is a chart of the various Light and Shadow powers, including their procs with and without this legendary ring equipped.

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SHADOW ABILITIES
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Tier 2 Trait

Without [item=152626] Equipped

With [item=152626] Equipped

[spell=252091]

Mastery increased by 500.
Avoidance increased by 500.

Mastery increased by 750.
Avoidance increased by 750.

[spell=252191]

Your Versatility is increased by 1,500 while Concordance of the Legionfall is active.

Your Versatility is increased by 2,250 while Concordance of the Legionfall is active.

[spell=252875]

Your spells and abilities have a chance to deal 200,000 Shadow damage and heal you for the same amount.

Your spells and abilities have a chance to deal 300,000 Shadow damage and heal you for the same amount.

[spell=252888]

Your spells and abilities have a chance to deal 60,000 to 300,000 Shadow damage and heal you for 60,000 to 300,000.

Your spells and abilities have a chance to deal 90,000 to 450,000 Shadow damage and heal you for 90,000 to 450,000.

[spell=252906]

Your spells and abilities have a chance to cause Torment, dealing 80,000 Shadow damage over 15 sec. This effect stacks up to 3 times.

Your spells and abilities have a chance to cause Torment, dealing 120,000 Shadow damage over 15 sec. This effect stacks up to 3 times.

[spell=252922]

Your spells and abilities have a chance to afflict the target with Sorrow, causing it to burst after 8 sec, dealing 186,350 Shadow damage.

Your spells and abilities have a chance to afflict the target with Sorrow, causing it to burst after 8 sec, dealing 279,525 Shadow damage.

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LIGHT ABILITIES
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Tier 2 Trait

Without [item=152626] Equipped

With [item=152626] Equipped

[spell=252088]

Haste increased by 500.
Movement speed increased by 500.

Haste increased by 750.
Movement speed increased by 750.

[spell=252207]

Your spells and abilities have the chance to cause Refractive Shell, absorbing 300,000 damage. Lasts 10 sec.

Your spells and abilities have the chance to cause Refractive Shell, absorbing 450,000 damage. Lasts 10 sec.

[spell=252799]

While Concordance of the Legionfall is active, your critical strike is increased by 1500.

While Concordance of the Legionfall is active, your critical strike is increased by 2250.

[spell=253070]

Your harmful spells and abilities have the chance to deal 135,000 additional damage and grant you Holy Bulwark, absorbing up to 135,000 damage over 8 sec.

Your harmful spells and abilities have the chance to deal 202,500 additional damage and grant you Holy Bulwark, absorbing up to 202,500 damage over 8 sec.

[spell=253093]

Your harmful spells and abilities have the chance to deal 101,000 additional Holy damage. Your helpful spells and abilities have a chance to heal for 101,000.

Your harmful spells and abilities have the chance to deal 151,500 additional Holy damage. Your helpful spells and abilities have a chance to heal for 151,500.

[spell=253111]

Taking damage has a chance to grant you Light's Embrace, restoring 45,500 health over 6 sec. Light's Embrace may stack up to 5 times.

Taking damage has a chance to grant you Light's Embrace, restoring 68,250 health over 6 sec. Light's Embrace may stack up to 5 times.

If you forget to grab the quest before killing Argus for the week. You can simply kill him a second time during the same lockout and get completion for the quest.