Chapter V: Judgment of the Black Prince

This achievement follows on from Chapter IV: Celestial Blessings, and the Judgement of the Black Prince story-line has been added during the Siege of Orgrimmar (when MoP launched there were only 4 chapters depicted on the Quests achievement page).
Some new quests have been discovered! The current information is below:
Meet Me Back at the Inn (after completing the previous Chapter IV quests)
A Timeless Discovery-journeying to the new zone called the Timeless Isle
Secrets of the Timeless Isle-used to be 100 (too easy for a legendary quest!), but is now 5,000 Timeless Coins. The coins can drop from mobs, looted from chests, participating in events and one-time quests (as well as a daily- A Timeless Question) and many other ways, so the grind shouldn't be too taxing.
The Emperor's Way-bring a raid to defeat the August Celestials, located at the Celestial Court. Related achievement: Celestial Challenge.
A Pandaren Legend-most likely rewarding you with the upgraded legendary cloak of your choice:
[item=98148] - [item=102248]
[item=98147] - [item=102249]
[item=98149] - [item=102247]
[item=98335] - [item=102245]
[item=98146] - [item=102250]
[item=98150] - [item=102246]
Judgement of the Black Prince-a follow-up quest granting the Chapter V achievement.

After obtaining your legendary cloak, it grants you access to engage Ordos, a world boss located on the Timeless Isle, a new zone implemented in the Siege of Orgrimmar patch. As long as one person has the cloak on the account, alts can then join the fight regardless of having completed Chapter V.
Source: http://us.battle.net/wow/en/forum/topic/9377568873?page=3#52

Related guild achievement: Shroud of the Celestials - Guild Edition

fwiw, upon completion, if you choose to follow wrathion to see where he goes, you can skip it. He will travel at approx. walking speed to the edge of the zone and phase out. thought this might save someone a few minutes.

Once 6.0 launches, It will no longer be possible to start Wrathion's Legendary Questline.. If you have not completed the Legendary Questline by the time Warlords of Draenor launches then you will no longer be able to progress on it and all Quests & Items will be removed.

All of the Achievements tied to the Questline will be turned into Legacy achievements, The "Judgement of the Black Prince" achievement will give a title "Legend of Pandaria"

Blizzard's reasoning behind this decision is that the Legendary Cape is a reward for following the content of Mists of Pandaria, They want it to be a way to show that you followed the content of Pandaria, and be a permanent recognition of your character's time and experience in the expansion.

Also stated, Is that the Warlords of Draenor Legendary Ring will likely follow a similar concept of being an item that you will work towards across your adventures in Draenor, but will then be unobtainable once we go into the expansion after.