Repaid Debt

Kalecgos is kind of a ^&*!. He asks you to go with him and speak to a guy in Shal'Aran, then teleports there without you. I don't believe a dragon (ex-)aspect of magic doesn't know how to create a portal.

Introduction
This quest is part of a comment-size guide for "Balance of Power." questline. Quests are pretty much self-explanatory and all it takes is some time and good will to complete them. Completing this questline rewards Artifact Appearance (Appearance 3, Tint 1).

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Part 1
Image of Kalec appears in your Order Hall and gives you the following quest:
The Power Within (Meet Archmage Kalec at Azurewing Repose in Azsuna.)
Reasons behind appearance of Archmage Kalec in your Class Order Hall are not yet completely understood but most people noticed him few days after they completed their Class Campaign.

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Part 2
From there you take a next quest from Senegos:
The Heart of Zin-Azshari (Retrieve the Heart of Zin-Azshari and defeat the Wrath of Azshara in the Eye of Azshara on Mythic difficulty.)

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Part 3
After completing the previous quests you receive a few new quests (exactly 3: two from Senegos and one from Archmage Kalec).
Senegos:
[1] A Vainglorious Past (Obtain a Vainglorious Draught from Veridis Fallon.)
- You can purchase Vainglorious Draught from Veridis Fallon in Crumbled Palace [46.9 , 41.4] in Azsuna for 150g (Requirement: Court of Farondis - Honored).

[2] Fallen Power (Recover a Corrupted Essence from Oakheart and defeat the Shade of Xavius in Darkheart Thicket on Mythic difficulty.)

Archmage Kalec:
[3] Tempering Darkness (Obtain 30 Blood of Sargeras.)
Should not be a problem since by the time you get that quest and if you did not spend any Blood of Sargeras you should have some stacked up simply by completing Dungeons and World Quests.

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Part 4
Archmage Kalec:
Lucid Strength (Absorb the power from the Corrupted Essence using the Heart of Zin-Azshari.)
Right next to you.

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Part 5
Senegos:
The Wisdom of Patience (Await the opening of The Emerald Nightmare, then report back to Senegos in Azsuna.)

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Part 6
Archmage Kalec:
[1] In Nightmares (Obtain a Deathglare Iris and the Horn of the Nightmare Lord from The Emerald Nightmare.) NOTE: NOT doable with LFR. It has to be Normal difficulty or higher. In order to complete this quest you need to kill 2 bosses: Il'gynoth and Xavius and loot mentioned quest items.

Senegos:
[2] Essence of Power (Collect 30 Corrupted Essence from The Emerald Nightmare.) This quest gave me a weird WoD and Khadgar flashback. NOT doable with LFR. Only Normal or higher difficulty. Essences do not have a 100% drop rate and bosses can drop 1-2 3 Corrupted Essences at the time.

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Part 7
Senegos:
Essential Consumption (Absorb the combined power of the Corrupted Essences using the Heart of Zin-Azshari.)
Right next to you.

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Part 8
Archmage Kalec:
Repaid Debt (Meet Archmage Kalec in the Shal'Aran in Suramar.)

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Part 9
Thalrenus Rivertree:
Lost Knowledge (Obtain the Scroll of Elun'dris from First Arcanist Thalyssra.)
- Costs 120g. Requirement: The Nightfallen - Revered).

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Part 10
Thalrenus Rivertree:
[1] Borrowing Without Asking (Recover a Containment Crystal from the Vault of the Wardens on Mythic difficulty.)

Syrana Starweaver:
[2] Rite of the Captain (Take Advisor Vandros' Eon Winder from The Arcway on Mythic difficulty.)

Lothrius Mooncaller:
[3] Literary Perfection (Find Lothrius' first edition copy of "Wards, Sigils, and the Nightborne Way" in the Court of Stars on Mythic difficulty.)

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Part 11
Syrana Starweaver:
Twisted Power (Use a Greater Portal Stone to summon Vizuul the Twisted at the Altar of End Times in Faronaar and collect his runebindings.)

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Part 12
Syrana Starweaver:
A True Test (Absorb the power from the Twisted Runebindings using the Heart of Zin-Azshari.)
Right next to you.

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Part 13
Archmage Kalec:
Seeking the Valkyra (Seek out Ashildir at the Vault of Eyir in Stormheim.)

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Part 14
Ashildir:
The Mark (Prove your worth to the Valarjar, then receive the Mark of the Valkyra from Eyir.)

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Part 15
Ashildir:
Retrieving the Svalnguard (Retrieve the Svalnguard and defeat Helya in the Maw of Souls on Mythic difficulty.)

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Part 16
Ashildir:
[1] A Feast Fit for Odyn (Acquire flank meat from Thunderhoof in Stormheim, Spinesever's spine in Highmountain, and a steak from Leytusk in Azsuna.)

[2] Presentation is Key (Retrieve 4 Adamantium Casing Scraps and defeat Dargrul the Underking in Neltharion's Lair on Mythic difficulty.)

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Part 17
Ashildir:
Odyn's Blessing (Obtain Odyn's blessing in the Halls of Valor on Heroic or Mythic difficulty.)
- Use Grand Feast of Valhallas when you enter the dungeon - at the start. You can enter the dungeon alone, put the feast down, get out and then get a group to defeat Odyn and get his blessing. After you defeat him wait for him to finish his speech (RP) and then you can collect his blessing on the loot chest.

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Part 18
Archmage Kalec:
Planning the Assault (Await the opening of the Nighthold, then meet Archmage Kalec in Shal'Aran.)

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Part 19
Archmage Kalec:
[1] Into the Nighthold (Collect 20 Nightshards from the Nighthold on Normal difficulty or higher.)
- Try and complete this quest without killing Gul'dan because if you kill him before acquiring The Nighthold: Darkness Calls quest in Part 20 he won't drop quest item for you this week and you'll need to wait for a next reset to complete The Nighthold: Darkness Calls quest in Part 20.

Thalrenus Rivertree:
[2] The Nighthold: Delusions of Grandeur (Collect the core of Trilliax and Grand Magistrix Elisande's Millennia Tome in the Nighthold on Normal difficulty or higher.)

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Part 20
Archmage Kalec:
The Nighthold: Darkness Calls (Bring one Gul'dan's eyes to Archmage Kalec on Normal difficulty or higher.)

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Part 21
Archmage Kalec:
Balance of Power (Absorb the combined power of the Nightshards and the Eye of Gul'dan using the Heart of Zin-Azshari at the Lunar Crucible.)

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Congratulations on completing this questline and acquiring new Artifact Appearance. Enjoy!
Foxies, out.

There are several tertiary requirements that you'll need to meet to be able to progress along the Balance of Power quest chain at various points. You can and should work towards completing these as soon as possible if you want to complete the quest chain quickly. If you wait to pursue these objectives until you're forced to by a quest, you'll increase your total time required to complete this chain significantly, as, especially in the case of Reputations, progression is time gated.
In the order they become relevant, they are:

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• Complete your Order Hall Campaign through A Glorious Campaign, Forged for Battle, and Roster of Champions. This is required for Kalec to appear in your Order Hall and offer The Power Within. It usually takes him 1-3 days after you've done this to appear.
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• Complete the "Behind Legion Lines" quest objective of Azsuna Matata, beginning with Into the Fray and ending with Dark Revelations to unlock and then complete the "Defending Azurewing Repose" quest objective of Azsuna Matata, beginning with Journey to the Repose and ending with Hunger's End in Azsuna. This is required before you can turn in The Power Within to begin the Balance of Power chain in earnest.
• 
  
• Earn Honored Reputation with the [faction=1900]. This is required to purchase the [item=139631] from [npc=107376] for A Vainglorious Past. This is the last tertiary objective you'll need to have done before you can begin Emerald Nightmare for your 30 [item=139706] for The Emerald Nightmare: Essence of Power. which will take you at least two raids to do unless you are extremely lucky.
• 
  
• Complete the "Nightfall" quest objective of Nightfallen But Not Forgotten, beginning with Khadgar's Discovery and ending with Arcane Thirst, then complete Shal'Aran to unlock and then complete the "Moon Guard Stronghold" quest objective of Good Suramaritan, beginning with Not Their Last Stand and Fate of the Guard, and ending with Last Stand of the Moon Guard in Suramar. This is required before you can complete Repaid Debt and progress the Suramar portion of the quest chain.
• 
  
• Earn Revered Reputation with [faction=1859]. This is required to purchase the [item=139780] from [npc=97140] for Lost Knowledge which you'll need to do as soon as you begin the Suramar portion of the quest chain.
• 
  
• Earn Revered Reputation with the [faction=1948] to prove your worth to Eyir and receive the Mark of the Valkyra for The Mark in Stormheim. This is the last tertiary objective you'll need to complete, and is required before you can progress the Stormheim portion of the quest chain.
• 
  

Lastly, while not a requirement, it is recommended that if you manage to collect all of the [item=140199] for The Nighthold: Into the Nighthold before you kill Gul'dan, that you return to Shal'Aran as soon as you have them and The Nighthold: Delusions of Grandeur completed, because you will not be able to loot the [item=140198] during the same week for the next and final quest, The Nighthold: Darkness Calls, if you have already killed him.

Edit: As of Patch 8.0.1, The Nighthold: Darkness Calls appears to be available alongside The Nighthold: Into the Nighthold and The Nighthold: Delusions of Grandeur instead of being made available after them.