WoWDB

This achievement is earned by wearing one of the new costumes added in 2014 for the  Day of the Dead.

The costumes you can purchase during  Day of the Dead for this achievement are:

•  "Snowy Owl" Contender's Costume
•  "Blooming Rose" Contender's Costume
•  "Night Demon" Contender's Costume
•  "Purple Phantom" Contender's Costume
•  "Santo's Sun" Contender's Costume

After you complete  To The Afterlife, you open up the achievement  Vientos! which leads to  Calavera.

A way to cheese the achievements is to form a group with another interested player and go inside  Siege of Orgrimmar. You will not need to endure the resurrection timer and you can instantly resurrect yourself. It's still tedious, but not anywhere near as frustrating.