This quest is marked as a raid; however, it can easily be soloed if you desire and don't have the time to do it with a group of friends. Bolvar does all of the fighting, so just stand back away from him and the dragons. Take some bandages, a healthstone, or health potion just in case one of the dragons hits you. If you have a ranged attack you may use it to ""tag"" the kills and gain XP if you're not already 60.

As a side note: Windsor is not attacked as you escort him to SW Keep.

On beginning the quest:

Lady Katrana Prestor yells: Seize him! Seize the worthless criminal and his allies!

Windsor beings walking, and reaches Marcus Jonathan, who has formed a line with his guards to block Windsor.

General Marcus Jonathan says: Reginald, you know that I cannot let you pass.
Reginald Windsor says: You must do what you think is right, Marcus. We served together under Turalyon. He made us both the men that we are today. Did he err with me? Do you truly believe my intent is to cause harm to our alliance? Would I shame our heroes?
Reginald Windsor says: Holding me here is not the right decision, Marcus.
General Marcus Jonathan appears lost in contemplation.
General Marcus Jonathan says: I am ashamed, old friend. I know not what I do anymore. It is not you that would dare bring shame to the heroes of legend - it is I. It is I and the rest of these corrupt politicians. They fill our lives with empty promises, unending lies.
General Marcus Jonathan says: We shame our ancestors. We shame those lost to us... forgive me, Reginald.
Reginald Windsor says: Dear friend, you honor them with your vigilant watch. You are steadfast in your allegiance. I do not doubt for a moment that you would not give as great a sacrifice for your people as any of the heroes you stand under.
Reginald Windsor says: Now, it is time to bring her reign to and end, Marcus. Stand down, friend.
General Marcus Jonathan says: Stand down! Can you not see that heroes walk among us?
General Marcus Jonathan says: Move aside! Let them pass!
General Marcus Jonathan yells: Reginald Windsor is not to be harmed! He shall pass through untouched!
General Marcus Jonathan says: Go, Reginald. May the light guide your hand.

Next, he walks through Stormwind, to the keep. As he passes guards, they will say one of a few things:

""Light be with you, sir.""
""We are but dirt beneath your feet, sir.""
""...nerves of thorium.""
""Make way!""
""A living legend...""

He stops at the entrance to the keep:

Reginald Windsor says: Be brave, friends. The reptile will thrash wildly. It is an act of desperation. When you are ready, give me the word.

You will have to talk to him to proceed. His talk box says:

The dragon hides behind a royal facade. The tablets will reveal her for what she is... If what I have seen holds true, I will not survive this battle, friend. It is you that must finish what I have started. Are you forces ready? Are you ready? We will begin on your command.

The chat option is: I am ready, as are my forces. Let us end this masquerade!

He walks up the hallway to the throne room, and confronts Lady Prestor.

Reginald Windsor says: Majesty, run while you still can. She is not what you think her to be...
Highlord Bolvar Fordragon says: To the safe hall, your majesty.
(Anduin Wrynn leaves)
Reginald Windsor says: The masquerade is over, Lady Prestor. Or should I call you by your true name... Onyxia...
Lady Katrana Prestor laughs.
Lady Katrana Prestor says: You will be incarcerated and tried for treason, Windsor. I shall watch with glee as they hand down a guilty verdict and sentence you to death by hanging...
Lady Katrana Prestor says: And as your limp body dangles from the rafters, I shall take pleasure in knowing that a mad man has been put to death. After all, what proof do you have? Did you expect to come in here and point your fingers at royalty and leave unscathed?
Reginald Windsor says: You will not escape your fate, Onyxia. It has been prophesied - a vision resonating from the great halls of Karazhan. It ends now...
Reginald Windsor says: The Dark Irons thought these tablets to be encoded. This is not any form of coding, it is the tongue of ancient dragon.
Reginald Windsor says: Listen, dragon. Let the truth resonate throughout these halls.
(he begins a casting animation, similar to the animation for the paladin spell Redemption)
Reginald Windsor reads from the tablets. Unknown, unheard sounds flow through your consciousness.
Highlord Bolvar Fordragon gasps.

He finishes the casting, and Lady Prestor becomes a black dragon. Eight level 60 elite dragonspawn ""https://vanillawowdb.com/?npc=12739 Onyxia's Elite Guard"" appear around the room.

Lady Onyxia says: Curious... Windsor, in this vision, did you survive? I only ask because one thing that I can and will assure is your death. Here and now.
Highlord Bolvar Fordragon yells: Dragon filth! Guards! Guards! Seize this monster!
Reginald Windsor says: DO NOT LET HER ESCAPE!

As fighting begins, Lady Onyxia hits Windsor with a lightning spell, causing him to collapse. She then teleports away, disappearing. This emote also occurs:

Highlord Bolvar Fordragon's medallion shatters.

Once the dust settles...

Highlord Bolvar Fordragon says: Reginald... I... I am sorry.
Reginald Windsor says: Bol... Bolvar... the medallion... use...
Reginald Windsor dies.

Bolvar will then have the next step of the chain. Unfortunately, the quest chain will be removed for Wrath of the Lich King, because Varian Wrynn will return, and Bolvar and Katrana will leave. A shame too, it's simply an amazing questline.

I wish this quest line was still in the game but the phasing engine would be used so that people who've already done the chain wouldn't keep seeing Lady Prestor in the throne room.

It is tragic that Blizzard chose to remove this quest chain from the game in patch 3.0.2. It is an incredible part of the lore and a challenging chain. Some of us are not playing for just the end-game content, but to experience as much of the content of the game as possible, especially the parts with essential lore.