This quest is only available during the Lunar Festival (around the Chinese New Year), and can be repeated every year.
The beast Omen was once a hero of this world. A wolf of great strength and fortune, Omen fought beside the heroes of the War of the Ancients until he succumbed to demonic magics. Turning against his allies, he raged across Azeroth and carried death and terror in his wake.
Gather cohorts, <Name>, and summon and defeat the beast Omen. Earn the blessing of Elune!
Very well done, <name>. Defeating Omen sent his spirit back to the waters of Elune'ara. Although the beast himself is blessed by Elune and so cannot die, his anger will subside for another year.
Your victory is a celebration, <name>--a celebration of the might of Omen, of you and your allies, and of the peace you have given this tragic hero of our past.
You do not need to be in the raid that taps and kills Omen, you just need to stand in the light that shines on his dead body to receive Elune's Blessing and complete the quest. (So if you see folks fighting him, don't worry about raid invite, just lend a hand.) It's considered rude not to help if you want to do this.
You need to be at least Level 40 to get the quest. Up until Level 60, use Elune's Candle (from the quest giver) to take 1K health off him from each charge used. Note you can remove most of your gear if you wish to lessen repair bills.
Omen has 5570K Hit Points and he hits VERY hard. When the quest says to bring your friends, BRING YOUR FRIENDS! Better yet, make some new friends and bring them too! Once you are in the area (65,54) you need to be patient until he appears. I waited for about 15 minutes and then he came out of the lake.