Korrak the Bloodrager

If you are having trouble getting this completed, go to old dalaran, then zul'drak (spot in the map), complete the quests there in the amphitheatre.
Korrak is the last one and it gives credit for the event quest.

If you don't wanna bother doing PvP in Alterac Valley, you can simply solo this quest by heading to Zul'Drak at 48, 56, talk to [npc=30007] and pick up the quest The Amphitheater of Anguish: Yggdras!
Then, continue with the following quests:
- The Amphitheater of Anguish: Magnataur!
- The Amphitheater of Anguish: From Beyond!
- The Amphitheater of Anguish: Tuskarrmageddon!
- and finally, The Amphitheater of Anguish: Korrak the Bloodrager!. This will spawn Korrak and killing him will give you both credit for the arena quest and the anniversary quest. It is unknown if it's intentional or not.

Note that you have to be minimum level 64 to start these quests and the bosses scale up to level 80. If you are under level 80, you will most likely need a higher level friend to help you, otherwise, I was able to easily solo these bosses at level 90.

If you've already completed this questline in the past, and need to summon Korrak again, you will have to find someone that haven't done the quests to pick them up for you so that you can tag the bosses. EDIT: You apparently won't get credit if you spawn Korrak using Party Sync. Make sure to turn it off when Korrak spawns then kill him.

Final note, if you're leveling and plan on reaching level 120 by the end of the event, make sure to save this quest until you're max level so you can receive a 400ilvl weapon as soon as you reach max level. Good luck and happy 15th anniversary!

Ok, here is a cheap way to complete this. It's true that if you kill the Korrak in the Zul'Drak amphitheater that you will get credit for this quest. But what if you don't have access to the Amphithear series because you already did it years ago?
The answer is: Party Sync.
Party sync lets you repeat old quests, including the Amphitheater series.
Next problem: Using Party Sync switches off your current Quest list (which means if you kill korrak with party sync on, it won;t give you credit). How do you get around this?
The answer: Run through the amphitheater until you get to korrak. Start the korrak quest so that he spawns and enters the arena. Once he becomes killable, end your party sync. With party sync ended, you can immediately kill him and it will count. (Tested, by me, and it works.)