Defeat MOTHER in Uldir after entering her secret passcode on Normal difficulty or higher.
To complete this achievement you have to enter the correct passcode during the MOTHER boss fight. The passcode is introduced by clicking a sequence of 3 buttons that you can find in either side of each of the 3 rooms (both buttons in each room are essentially the same one). The passcode is randomized every time, leaving 6 different possible combinations (123, 132, 213, 231, 312, 321). To figure out the passcode, you need to do the following:
The best way to do this is to send a few DPS and a healer to the second or the third room to check the buttons.Do all the steps, defeat the boss and the achievement is yours.
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Full Guide: https://www.mmo-champion.com/content/8274-Glory-of-the-Uldir-Raider-Achievement-Guide