You knew it would eventually appear on the front page. Here is a mini-preview of what to expect of the Lich King encounter/storyline in the Icecrown Citadel.

The fight against the Lich King

There isn't many details on this fight, I'm not really holding back any information it's just that most of the elements aren't implemented yet.
The last screenshot of the Frozen Throne I posted was on the first day of PTR and showed a very early version of the Throne. The model was eventually updated and this is how it looked in later PTR builds:

I'm not sure if this is the final model, but it looks better than what I posted a few months ago. The fight will also most likely feature the "Frostmourne" room of the citadel, it was also slightly modified since the last time I posted. (The video is the early version, the screenshots are from PTR builds released a few weeks later)

The only thing we know about the fight is from the Blizzcon where it was revealed that a part of the Frozen Throne will collapse after each phase. The screenshots below shows the throne before / after the 4 external parts collapsed.

And that's it for the fight, we'll probably have to wait for the patch before we get more information on that.

The death of the Lich King

Yup. Sorry. The Lich King is going to die. Blizzard didn't have any problem killing Kael'thas, Illidan, Kael'thas, Lady Vashj, or even Kael'thas and it won't be any different for our old friend Arthas.
It doesn't mean that the Frozen Throne will stay empty, Arthas will be replaced and a new character will take his place ...

• HR_Uther_Alliance06 - There is... something else that you should know about the Lich King. Control over the scourge must never be lost. Even if you were to strike down the Lich King, another would have to take its place. For without the control of its master, the scourge would run rampant across the world, destroying all living things.
• HR_Uther_Alliance07 - A grand sacrifice by a noble soul.
• HR_Uther_Alliance08 - I do not know, Jaina. I suspect that the piece of Arthas that might be left inside the Lich King is all that holds the scourge from annihilating Azeroth.

There isn't any official information about that, it is commonly admitted that Bolvar Fordragon will step up and become the new Lich King to prevent the destruction of Azeroth by the Scourge.

The rumor first appeared when Blizzard pushed a new model named "Lavaman" in the PTR files, showing a character identified by many users as Bolvar Fordragon (the Immolated Champion in the Yogg-Saron event of Ulduar)

Blizzard eventually "removed" the model from later PTR builds by patching the files to make them blank (= 0 Byte).

Conclusion

It's time for another wave of people insulting me! If you're still reading this news I will assume that you do not care about spoilers.

• Yes, Bolvar is the new Lich King.
• Patch 3.3.x (most likely 3.3.2) should be released before the end of January, it will also include the cinematic cutscene mentioned earlier and the ending of Icecrown Citadel will be spoiled at this point.
• This patch will also include the Season 8 Gear, if Season 7 lasts 17~18 weeks like the 3 previous seasons you can expect the patch mid-january
• Blizzard usually starts their Arena Season a few weeks before the last tier of PvE content is unlocked. Expect to fight Arthas in early/mid February

Once again, there is nothing official here and I could be wrong and I'm sure it will eventually happen at some point. Hopefully I'll get lucky once again.
Also please refrain from discussing this on the official forums to not spoil the event for people who want to discover things by themselves.
Small reminder, Invincible's Reins will most likely drop from the Lich King. (Heroic Mode, probably)