Cutting Edge: Lei Shen

These "Cutting Edge" achievements are Blizzards solution to raiders feeling like their accomplishments were cheapened when a few patches later, players could go in and easily defeat bosses that once caused endless hours of frustration. The only solace raiders previously has was "Check my achievement date, I got that so far before everyone," while now these provide a small reward for those who managed to clear heroic end bosses when they were current.

So far we have three achievements:
Cutting Edge: Will of the Emperor
Cutting Edge: Grand Empress Shek'zeer
Cutting Edge: Sha of Fear

And it looks like Blizzard will continue this trend in 5.2 with this one as well as Cutting Edge: Ra-den

In 5.2 Blizzard will also be releasing "Ahead of the Curve" achievements for those that managed to clear end bosses on normal when they were current content.
Ahead of the Curve: Will of the Emperor
Ahead of the Curve: Grand Empress Shek'zeer
Ahead of the Curve: Sha of Fear

And again, Blizzard will continue with these achievements in 5.2 with Ahead of the Curve: Lei Shen

These achievements will be phased out over time, becoming unattainable when the new raid tier is released. Meaning that after 5.2 is released these achievements (Normal or Heroic) for Will of the Emperor, Grand Empress Shek'Zeer, and Sha of Fear will no longer be attainable.

Note that the titles are not linked to these achievements so if you didn't manage to get Delver of the Vaults or The Fearless, you can go back and get them later!

To repeat for clarity, there's reportedly no new raid in Patch 5.3, which probably means Blizzard are continuing the pattern of raids then a smaller patch:
Mists of Pandaria -ToES, HoF and MV raids (5.0.4)
Landfall-Arrival on Pandaria of the Alliance and Horde (5.1)
Discovery of the Throne of Thunder raid and Isle of Giants (5.2)
5.3 will not have a raid, as evidenced here:
http://us.battle.net/wow/en/forum/topic/7592242035#1.
So for those that are worried about rushing this, there will be ample time to defeat Lei Shen over two patches.

For people surprised on not being able to find this achievement in their book, sometime in 6.0 they merged the leishen achievements into the raden cutting edge achievement, if you look under your raden achievement you can see your 2 missing leishen achievements.