During Veteran Ruins of Mazzatun, there will be a room with 6 elder NPC's- right before the final boss in the dungeon. This room will have a "Calendar" you can activate in the middle
Upon activating it there will be a glowing green symbol on the ground - a player needs to find the corresponding cage with the symbol, and activate it. You can only activate it after the adds are cleared, and there is a time limit to do so before the elder dies. The easiest way to do this is have 3 members on your team handle the mobs, and 1 player dedicated to watching the symbols and waiting by the correct cage to activate it right after the adds are cleared. As the achievement description states, this must be done on Veteran to unlock.
Here is a video guide on how the achievement is done, all credit goes to xxDAMRONxx
https://www.youtube.com/watch?time_continue=3&v=biHakuBK...