Simply head to the Vale of Eternal Blossoms, Townlong Steppes, or the Dread Wastes and kill level 90 mantid and mogu. Each kill will grant you reputation with Wrathion. Don't worry, it's not that bad of a grind, especially since all of these creatures are involved in the various daily quests you'll be doing for faction reputation after you've hit level 90. It's a fairly natural progression, and it shouldn't take long to knock it out.
Once you've reached honored reputation, head back to Wrathion. Since you've shown him you're serious about helping him out, he's got another task for you. This time, you'll need to gather ten Sigils of Power and Sigils of Wisdom and return them to the Black Prince. These items are only gathered by killing raid bosses in Mogu'shan Vaults, Heart of Fear, or Terrace of Endless Spring -- so this is where raiding comes into the picture.
This doesn't necessarily mean you need to be in a raid guild. In fact, you can gather the Sigils from raid finder mode. However, it is much, much slower when you go that route -- so if you'd like to finish the quest speedily, you may want to look into joining a raiding guild. After collecting all of the items and returning to Wrathion, he will send you to kill the Sha of Fear in Terrace of the Endless Spring, get your hands on the Chimera of Fear, and return it to him.
The rewards for this chain are not weapons. They are crystallized items that presumably will be used to modify sha-touched weapons somehow. Beyond this stretch of the chain, we know very little of what Wrathion will be doing ... but it's apt to be a legendary experience. Don't forget about Wrathion once you've hit level 90, you can find him sitting quietly in the Tavern of the Mists, located at the top of the Veiled Stair.
It's open warfare between Alliance and Horde in
Mists of Pandaria,
World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!