What the achievement asks
Buy the Crimson Tidestallion from Mrrl. Five points, account-wide.
The credit lands the moment you buy the mount, not when you learn it. One purchase, one achievement. Getting to the point where you can make that purchase is the entire difficulty.
Two things have to be true before the mount will appear
You must be wearing the Azsh'ari Stormsurger Cape. Mrrl keeps a hidden stock, and the Crimson Tidestallion is only visible to a character wearing that cloak. Without it you will stand in front of him and see an ordinary shop.
The cape comes out of a Benthic Cloak, bought at your faction's Nazjatar base for five Prismatic Manapearls. Which cloak you get is random, so budget for several attempts.
And his stock has to be offering it. Mrrl's hidden wares rotate. Even correctly dressed, some days the mount simply is not there, and the answer is to come back tomorrow.
Buying a different secret item from Mrrl removes the Tidestallion from your view. If you are here for the achievement, buy nothing else from his hidden stock until the mount is yours.
The trading chain
The mount is not bought with gold. It is bought with a Hungry Herald's Tentacle Taco, and that taco sits at the end of a chain of murloc junk, each step consuming the last.
The whole thing costs roughly eighteen hundred gold and forty Prismatic Manapearls, and takes about a quarter of an hour once you know the order. The four murloc traders stand within sight of each other in your camp.
| Step | Trader | Buy |
|---|---|---|
| 1 | Hurlgrl | 128 Sweet Sea Vegetable |
| 2 | Mrrglrlr | 714 Slimy Naga Eyeball |
| 3 | Flrgrrl | 288 Unidentified Mass |
| 4 | Flrgrrl | 32 Just Regular Butter |
| 5 | Grrmrlg | 238 Curious Murloc Horn |
| 6 | Hurlgrl | 48 Dirty Murloc Sock |
| 7 | Hurlgrl | 14 Healthy Murloc Lunch |
| 8 | in the water | Click each sock to make 48 Clean Murloc Sock |
| 9 | Flrgrrl | 28 Ghost Food |
| 10 | Grrmrlg | 16 Sea Giant Foot Dust |
| 11 | Mrrglrlr | 16 Smelly Pile of Gloop |
| 12 | Mrrglrlr | 2 Pulsating Blood Stone |
| 13 | Hurlgrl | 4 Cultist Pinky Finger |
Then rescue Murloco, who is trapped in a cave at 46.3 32.6. Freeing him starts a short fight against about five waves of naga. He then sells the Tentacle Taco for 666 gold, and Mrrl will take it.
Where everyone stands
Those coordinates are the Alliance camp. Horde traders carry the same names in the same order, at the equivalent spots in their own camp, and the sock-washing water is right where Flrgrrl swims.
Curious Murloc Horns stack to a hundred rather than two hundred, so buying 238 of them takes three purchases rather than two. Nothing has gone wrong.