Im a beta tester, and I know that its a cinematic from the quest line: Battle for the Undercity.

The Alliance have risen with the Horde to fight against Icecrown Glacier. That isnt Saurfang, its Saurfang the Younger, Saurfangs son (Real confusing isnt it?). Then the Warlock Commander (Wearing Tier 7 for Warlock) from the Forsaken attack Arthas and the Horde/Alliance. Far after the cinematic, Thrall, Jaina, Sylvanas and YOU (Its a DK quest) go into Undercity and kill Tichondrius.

Sylvanas invading her own city?
The Forsaken [Sylvanas/Undead] kill Saurfang the Younger, a member of their faction?

Saurfang was killed by Arthas.

Arthas =/= Forsaken

Arthas is Scourge
Sylvanas is Forsaken/Undead

Hmm... night elf...

So undead still kill orcs?

Warcraft 2: Beyond the Dark Portal (I think it was this one)

Alleria, Sylvanas' Sister, was introduced, and lost at Draenor.

Warcraft 3: Reign of Chaos

Sylvanas first introduced, was a High Elf for 3 missions untill finally slaughtered at Silvermoon by Arthas Menethil.

Warcraft 3: INTERMISSION (NOT A GAME):

Arthas Ressurected Sylvanas to be bound to the Lich King, like a 'slave'.

Warcraft 3: The Frozen Throne:

Sylvanas returns with Kel'Thuzad, meets up with the three Dreadlords: Balnazzar, Varimathras, and Detheroc, makes plans to kill Arthas.
Sylvanas Ambushes Arthas in the forests and stings him with a poisoned arrow.
Kel'Thuzad has Sylvanas back away into the forests, she declares: "This isn't over Arthas! I'll never stop hunting you!"
Sylvanas confronts Varimathras, Varimathras begs to live and betrays brothers.
Sylvanas Kills Detheroc
Sylvanas (Doesn't really) kill Balnazzar.
The two create The Forsaken.

WoW:

Sylvanas still hates Arthas, and then plots to strike him at Northrend.
Varimathras and the Apocothery plot a rebeliion against Sylvanas.
Forsaken strike at Icecrown, killing the Horde and Alliance in the aftermath.

Forsaken kill everyone because they're no longer "Horde" because they backstabbed Undead
Alliance for obvious reasons, and Arthas because, when you get down to the point, who likes Arthas?

Thank you for a very clarifying post.
Knew the lore somewhat, played the expansions, just couldn't fit it together.

Thanks!

Prince Arthas Menethil was the son of King Terenas II.

King Terenas was the King of Stormwind in the game Warcraft: Orcs vs Humans (THE VERY FIRST GAME)

King Terenas II gave birth to Arthas, who was training under the paladin Uther the Lightbringer.

Uther the Lightbringer was in Warcraft 2,s till a paladin of the Order of the SIlver Hand.

The Silver Hand was a paladin Order that was sacred throughout Lordaeron.

It was still the time of War between the Orcs and Humans, the orcs had been attacking Strahnbrad, and so Arthas split up from Uther to protoect the town, later after the attack, Arthas killed a Blademaster. THe Blademaster said something about Demons coming. Uther told Arthas that the Demons are long gone (Wc2 Reference). Arthas met up with Jaina Proudmoore (current ruler of Theramore, and old friend of Arthas). Arthas and Jaina confronted the scourge for the first time. Kel'Thuzad, who was once a Dalaran Wizard, is now a Necromancer who started the Cult of the Damned (Scourge). Arthas killed Kel'Thuzad, as the Lich King intended.

Arthas than became more darker and more vengeful after the missions in Warcraft 3: Reign of Chaos. Eventually, a dreadlord named Mal'Ganis attacked Stratholme, Arthas had to purge the city due to a plagued grain infection. Arthas tried to confront Mal'Ganis, but the dreadlord lured Arthas to Northrend.

Arthas went to Northrend, and found Muradin Bronzebeard, famous mountain king of Ironforge. Arthas was outnumbered by Mal'ganis, and looked for the runeblade Muradin mentioned to him: Frostmourne.

ACTUAL QUOTES FROM THE FROSTMOURNE SCENE:

Arthas: "Behold Muradin, our salvation... Frostmourne."
Muradin: "Hol' lad, theres an inscription on the dais, its a warning..."
Muradin: "It says..."
Muradin: "Whomsoever takes up this blade shall wield power eternal, just as the blade rends flesh, so must power, scar the spirit."
Muradin: "Oh I should of known! The blade is cursed! Let's get the hell out of here!-"
Arthas: "I would gladly bare any curse to save my people Muradin."
Muradin: *Sigh of caution* "Leave it be Arthas, forget this buisness and... lead your men home!-"
Arthas: "Damn the men! Nothing will stop me from having my revenge old friend... not even you.."
Arthas: "Now, I call out to the spirits of this place... I will gladly give anything... or pay any price... if only you will help me save my people..."
Frostmourne: *The Lich King chants through the runeblade, the ice prison of Frostmourne explodes, one piece stabbing Muradin in the stomach. Frostmourne falls before Arthas, and he takes the blade."

Arthas then finishes off Mal'Ganis' base, and heads for Mal'Ganis himself.

Mal'Ganis: "I see you wield the Runeblade Frostmourne, but at the cost of your friend?"
Arthas: "It doesn't matter now, I hear only the voice of Frostmourne.."
Mal'Ganis: "You hear the voice of the Dark Lord(Lich King), tell me prince, what does the Dark Lord tell you now?"
Arthas: "He says, my time for vengeance has come..."
Mal'Ganis: "What? He couldn't possibly mean-"
*Arthas slays Mal'ganis* "It is finished!"

Arthas then becomes corrupted by Frostmourne, he returns to Lordaeron, with the people cheering and raining flowers down on him, he reaches the Throne room of King Terenas II.

ACTUAL DIALOGUE:

*Arthas slams the door open and walks in to the center of the throne room, he unsheathes Frostmourne and kneels before King Terenas*

King Terenas: "Ah my son..."
Arthas: "..and you, no longer need to sacrifice for your people, you no longer need to bare the weight, of your crown.. I've taken care... of everything!"
*Arthas gets up and lowers his hood, he walks up to King Terenas and grips him by the collar*
Terenas: "What is this?- What are you doing my son..?"
Arthas: "Succeeding you... father!"
*Arthas stabs through the chest of King Terenas*.
Arthas: "Let your eyes close..."
Arthas: "This Kingdom! Shall fold! And from the ashes, shall arise a new order that will shake, the very foundations... of the world!"

Much later, Arthas is attacked by Sylvanas after the Dreadlord ambush, he retreats to Icecrown Glacier after the Lich King tells him to, he meets Anub'arak, who leads him through Azjol'Nerub. Illidan Stormrage (Duh who doesn't know him) then battles Arthas for the Frozen Throne, Illidan is humilated and retreats to The Black Temple. Arthas claims the Armor of the Lich King and sits upon the throne untill Wrath of the Lich King.

Added (08/11/2008, 6:02 Am)
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Wow did I really type all that? o.O

King of Ironforge dies?

Different names maybe?

But huge lore loop.

Oh it's Magni Bronzebeard.

Relative of sorts.