Hanging out in Moonglade, he's a real pain. Before you even think of killing him, you need to get to Moonglade first. Since I mainly roll with my warrior when attempting new things, I wasn't able to just take the easy way to Moonglade and teleport myself there. However, many thanks to Toast who filled me in on a little tip: talk to the Lunar Festival Night Elves in any major city to get a scroll that'll teleport you to Moonglade. To get the scroll all you have to do is complete a quest that has you light off a few fireworks that you can purchase from a near by vendor. Easy as pie.
Once you arrive in Moonglade, head up to the northeast corner of the map, in particular around coordinates 53, 35. You'll find the quest "Elune's Blessing" from Valadar Starsong, giving you the task of killing Omen for some handy rewards.
If you want to get him killed, be prepared to take 10 to 15 of your level 70 friends with you. Have enough healing to keep a (hopefully) well geared tank alive, and enough DPS to get him down fast without having to worry about your healers going out of mana. Like most boss mobs, he does a frontal cleave. His only other trick is an AOE spell that does about 1800 damage to everyone in reaches. If you manage to keep things going for the five minutes it takes to kill him, Omen will fall before you (although he doesn't give out any loot).
Of course if you're having trouble killing him, there is always the kiting method.
And lets say that you're just standing around watching other people kill Omen. While hopefully you'd lend a hand, technically you don't need to be in the raid group to complete the quest. You can just stand in the beam of light that appears over his dead body, and like magic you'll have finished the quest.
Let's just take one quick peek at a reward for killing him. Elune's Lantern really intrigued me when I found out about it today. (On a side note, that's one of the great things about this game... after playing it nearly every day since launch, I'm still finding new stuff!) With one piece of Solid Stone as the reagent, you can use Elune's Lantern once a day and create an Elune Stone. When you use the Elune Stone, you're presented with a green target. Click the target anywhere. You and everyone else will see a bright beam of white light shine down on the area, just like the one pictured below. I'm going to be putting this to immediate use when raiding this week, and mark certain key spots with it. Now... I just have to remember to make a new Elune Stone each day.
Enjoy your new lantern and killing Omen!