Before I was distracted, I was able to find out more about this Horde gathering. Sylvanas has summoned the leaders of the Horde and its allies. For what purpose, I cannot say.
We aren't in a position to strike her down--she is too well-protected. But perhaps we can learn something that will prove useful in the ongoing war.
Keep your head low and take a look. I'll keep an eye out for more guards.
I cannot help but pity Baine. Honor such as his has lost its value in the Horde.
Sylvanas has made a bold move. We'll go after her, but she's cunning. We need to push back the naga and gather our forces for a final strike.
Notes
Horde gathering witnessed
Use the SI:7 Spyglass beside Shaw to see what's going on down below.
(The scene drops into cutscene mode.)
Sylvanas is accompanied by Nathanos Blightcaller down to the center area of Warfang Hold, where the racial leaders, allies, and some of the greatest champions affiliated with the Horde wait at the circumference. Starting from the left:
Sylvanas begins without preamble as she approaches the bonfire in the center.
Lady Sylvanas Windrunner says: I have troubling news.
Lady Sylvanas Windrunner says: It seems there are traitors in our midst.
She continues addressing those she passes as she walks around the left of the fire, while Blightcaller walks around the other side.
Lady Sylvanas Windrunner says: Most of you need fear nothing.
She walks toward the middle of the semi-circle... where Baine happens to be.
Lady Sylvanas Windrunner says: But Derek Proudmoore didn't flee to Kul Tiras by himself. He had help. Didn't he...
From Baine's direction, she suddenly looks to his left, addressing Thomas Zelling.
Lady Sylvanas Windrunner says: ... Zelling?
Thomas flinches. He tries to speak, but stammers and is lost for words. His bow already having been in hand, Blightcaller nocks an arrow and pulls it to full draw at him. Thomas quietly gasps in fear.
Baine Bloodhoof says: Stop!
Baine firmly breaks the silence and Blightcaller raises his gaze.
Baine Bloodhoof says: I returned Derek Proudmoore to his family.
Thomas looks at Baine as he speaks. The High Chieftain continues on without fear.
Baine Bloodhoof says: You raised him as a Forsaken. But you planned to deny him his free will. To violate his mind.
Baine Bloodhoof says: I could not stand by and permit such depravity to unfold.
Sylvanas replies calmly.
Lady Sylvanas Windrunner says: Thank you for your honesty.
Without warning, Blightcaller fires; the arrow pierces Thomas through the chest and kills him instantly. Rexxar can be seen in the background reacting in shock. Blightcaller calmly nocks another arrow and aims at Baine, who is outraged at the execution.
Baine Bloodhoof says: Banshee!
As he addresses Sylvanas, some of the others (particularly out of Sylvanas' field of view) shuffle uneasily.
Baine Bloodhoof says: Are we nothing more than pawns in your game?
Baine Bloodhoof says: You betray the Horde!
Lady Sylvanas Windrunner says: No. He did. And so did you.
She turns her head to address the nine guards standing on the surrounding upper rise.
Lady Sylvanas Windrunner says: Take him.
The guards walk around the center to gather behind Baine. He gives her a warning.
Baine Bloodhoof says: Others will see you for what you are, Sylvanas.
Baine Bloodhoof says: You will leave the Horde in ruins!
He turns and leaves with the guards without resistance, who form up around him.
All eyes are on Sylvanas as she watches Baine being taken away for a brief moment. She speaks calmly, but deliberately.
Lady Sylvanas Windrunner says: There is no place for weakness in our ranks.
She turns around to look at them all.
Lady Sylvanas Windrunner says: Remember your loyalties.
Cutscene exit
Shaw speaks grimly.
Mathias Shaw says: That was ruthless, even by the Banshee's standards.
Mathias Shaw says: She did this to send a message to the rest of the Horde: Dissent will not be tolerated.
Mathias Shaw says: Report our findings to the Lord Admiral. I'll make my way back on my own.
Complete
Lady Jaina Proudmoore says: Baine showed compassion and honor by returning Derek to us. I fear that choice may cost him his life.
Lady Jaina Proudmoore says: For years I pitied Sylvanas for what Arthas did to her. But nothing remains of the hero she was. Perhaps now the Horde will see that, too.
Jaina concludes with the only solution clear to her with a note of finality.
Lady Jaina Proudmoore says: To have any hope of ending this war, we must end her.
Secret dialogue
After finishing this quest, you can come across a secret one-time interaction between Jaina and Tandred.
A Gathering of Foes
[Radiant Azerite Core] (750x 
[Azerite])
70
[60] Shaw's Shank Redemption
Patch 8.1.5 (2019-03-12): Added.