Proof and Promise

The first quest in this questline is Prophecy Stirs and can be picked up at 72.0, 44.5 in Orgrimmar for Horde players and 78.0, 37.3 in Stormwind for Alliance players. You must be level 70 to pick up this quest.

After completing the quest chain, a set of NPC "Ally of Arzaal" become available and will grant you a [item=208691] containing Argus crafting materials.

List of all allies of [npc=208894] and their locations:

Eranthi - Antoran Wastes (58.9, 23.7)
Heraat - Krokuun (56.4 ,52.2)
Saatilu - Krokuun (61.5 ,28.7)
Tronaar - Eredath (51.9, 67.9)
Yazdan - Antoran Wastes (53.8, 52.9)
Zyaana - Eredath (55.0, 27.8)

Just for lore and story.
Prophecy Stirs

Prophet Velen says: I am glad my message reached you, Funkowah.

Uncertainty

Prophet Velen says: In the time that has passed, Hatuun and his people have continued to reclaim Argus from the Legion. Perhaps he has made progress.
Prophet Velen says: Let Romuul know when you are ready to depart.
Grand Artificer Romuul says: Confirming beacon coordinates...
Grand Artificer Romuul says: Beacon locked. Initiating transport to Argus. Good luck, Funkowah.

After landing in Krokuun:

Prophet Velen says: Hatuun, it is good to see you well. I had been concerned that--
Chieftain Hatuun says: Spare me your talk of concern. I did not call you here for pleasantries.
Chieftain Hatuun says: I have made an ally. One who wishes to meet with you.
Prophet Velen says: I would be honored to meet with any friend of yours. Who is this ally?
Arzal'kal says: It is I.
Prophet Velen says: You have allied with the Legion?
Prophet Velen says: I would have given you any aid they could offer!
Chieftain Hatuun says: I expected these accusations.
Chieftain Hatuun says: Even after you fled Argus, never returning for those you left behind, you would accuse me of--
Arzal'kal says: Please.
Arzal'kal says: Please hear me. I-- I have risked everything to speak with you.
Chieftain Hafuun says: Speak with him, "Prophet." Velen peers at Hatuun for a long moment.

Opening Wounds

Arzal'kal says: I... am aware of how this appears, Prophet. I will try to be brief.
Arzal'kal says: I believed Kil'jaeden and Archimonde. Their promises of prosperity for our people. How could I not?
Arzal'kal says: And so when they demanded we pay the Legion's price, I trusted them. We all did.
Arzal'kal says: But I never knew that price would be paid in millennia of bloodshed and ashes.
Prophet Velen says: And yet still you paid it.
Arzal'kal says: I did. Kil'jaeden had a way of invading the mind. Any doubt would be known to him immediately.
Arzal'kal says: Doubt was betrayal in his eyes. And betrayal, as you know, meant death.
Arzal'kal says: And I feared death more than the price of my sin.
Arzal'kal says: In the end, some of us fled the Legion. But the damage was already done.
Prophet Velen says: What will you do now, Arzal'kal?
Arzal'kal says: I want to show that I am no longer a coward. I cannot undo my crimes, but I will give all I have to try.
Hatuun crosses his arms, staring intently at Velen.
Prophet Velen says: Very well. Show me, if it will ease your pain.

Contrition

Arzal'kal says: I will meet you outside the cult's encampment. Tread carefully.
Chieftain Hatuun says: So you will listen, but send Funkowah in your place when it is time to act?
Prophet Velen says: I cannot guide Arzal'kal to a path. He must choose it of his own will.

After meeting with Arzal'kal:

Arzal'kal says: We begin here, Funkowah.
Arzal'kal says: I tried once to reason with Nalridun, but he and his followers cannot be persuaded from their path.

While fighting Nalridun:

Speaker Nalridun says: So you would abandon Sargeras, Arzal'kal? The only one who ever fulfilled his promises to us?
Speaker Nalridun says: Sargeras... his voice... I cannot hear...
Arzal'kal says: You heard only what you wanted to, Nalridun. As did so many of us.

After finish Proof and Promise and get the Seeing Red achievement:

Prophet Velen says: It cannot be... and yet I behold it. A shard of the Ata'mal crystal. Brilliant Star, it was called.
Prophet Velen says: This shard was lost to me on Draenor, taken by the orcs. I had thought it gone forever.
Arzal'kal says: Nearly. When the orcs disposed of it after their corruption, Kil'jaeden reclaimed it for the Legion.
Arzal'kal says: Many seized it since the Deceiver's death. But I could not let it remain in their bloodied hands.
Arzal'kal says: I was willing to die for it. I sought out the krokul. And then I sought out you.
Prophet Velen says: There are no words to express my gratitude.
Arzal'kal says: I do not deserve your gratitude. I should have done this sooner.
Prophet Velen says: I understand. I also have been forced to make difficult choices. At Eredath and Shattrath.
Prophet Velen says: But forgiveness is not easily given. Too many have suffered.
Arzal'kal says: I do not expect forgiveness. I cannot be forgiven. Not in a thousand lifetimes.
Arzal'kal says: But if this twisted soul can still do any good, I will see it done.
Prophet Velen says: Others have proven stronger than their darkness. The orcs, death knights, and Illidari. Perhaps there is a path forward.
Prophet Velen says: There is much I would like to discuss with you, Arzal'kal, and others who believe as you do.
Arzal'kal says: Please, just... Arzaal. If I may return, I would have it be with the name I had when I left.
Prophet Velen says: Very well. Come, Arzaal.
Prophet Velen says: Hatuun, please think on my offer.
Hatuun eyes Velen seriously, then looks away with a wordless nod.
Chieftain Hatuun says: I... did not expect Velen to react this way. He did not have to.
Chieftain Hatuun says: Perhaps things are different now. Perhaps we both are. I will... allow myself to hope.
Chieftain Hatuun says: Before you leave, speak with Gaal, our blacksmith. We have some things of value if you have resources to trade.