Undersea Usurper

It looks like you have to get all the achievements on one character. Having credit for a few on one doesnt show up on other characters.

UPD: Got it! No idea what to do with it now, but still it's pretty unique and fun to have in your collection.

Note that the achievement and its parts are NOT ACCOUNT-WIDE, so they must ALL be completed on one character.

The hardest parts of the whole endeavor which you need to prepare for:

1. Anemonar (Coral Ancient rare). While not hard to kill by himself (some tanks / pet DPS can even solo him), spawning him can be frustrating. Trick is, it's very hard to pull and kite the Colossal ray alone as it disengages if it feels no players within a VERY short range of itself. We had to do it in 2 people: a tank and my warlock friend. I used Goblin Glider to shoot the Concentrated Flame, and then we took turns taunting/healing to kite the ray to the forest. The only spot where to pull is the extension above Horde town (pull in air, glide there). Monks (Zen Flight), Druids (chicken) and DH (Glide) are best for this task.

2. Blackscale. Not hard to kill, but the chance to spawn him is very low. I camped the Packmother rare for 3 days. It does not matter if you have looted her: you can kill her as many times as you wish per day (with 1 hour respawn of course).

3. Hydra Scale Matriarchs x3. Of the three, only one spawns at a time (chosen randomly), at fixed spots. They share the same 1.5 hours spawn timer. I camped the last one for 3 days as well.

4. Tidemistress. Not hard to spawn her (you just need to know exactly where the 5 eggs are), but they are not guaranteed to appear. Besides, a lot of players do not even know she exists!

5. Pylon number 5 for Trove Tracker. A mother of all QTE's in WoW. Trick is not to memorize the sequence, but to learn to ignore the red color and only cast the beams once the sparks go real close to the runes. So close you CAN safely destroy them once they touch it. As loony as it sounds, CLEAR YOUR MIND: I mean, chill, relax, go slowly and steadily, many times in a row until you can subconsciously press these buttons. Reaction speed is more important than anticipation, let your spinal cord act in your brain's stead.

6. Periodic Destruction. Although in 415+ all of them are soloable, it takes 13 weeks sharp. Make sure you triple check the ingredients (and, of course, have some to spare at all times).

7. Nothing To Scry About 100 chests. Can be very frustrating if you do it in the enemy-infested areas. However, western Naga camp, the area immediately to the west and north of it (excluding the murloc village), waterfalls, ruins and hydra breeding grounds are just perfect spots. Flying helps a lot, so you may actually get about 10-15 chests per Stone.

8. Mrrl's Secret Stash. Since you will use your Tacos for every Tome out there, a useful trick is to wait until the Crimson Stallion day, get a Taco, vendor it, buy another Taco, and then immediately buy the Stallion and buy back the vendored Taco. Another thing VERY few people seem to know is that you can Ctrl-right click a reagent and then TYPE the desired number you want to buy (up to 200 common, up to 100 any other at a time). It also helps to record the ingredients down, like: A = 3 B + 7 C = 21 D + 28 E = 84 F + 140 G.

Everything else is either story progression, brainless farming, exploration (unlock flying first, then just follow the guides) or patience game with dailies.

Aqua Team Murder Force (Alliance) or Aqua Team Murder Force (Horde) is also needed, not listed up there (as of now.)

That one alone makes it a 2,5-3 month achievement.
And there's Periodic Destruction too, which needs 13 weeks in total to be completed in full.

All to reach this sweet, side-walking thing: [item=169194] and "of the Deeps" title ;)