To activate eridian runes in The Demon in the Dark for Wren in Konrad’s Hold in Borderlands 3, go to the back right of the chamber and look at the eridian rune to the right of the door through the rings and shoot it.
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Go to a similar posiion on the left side of the chamber and look through the rings at an eridian rune further to the right of the door and shoot it.
Finally, climb up the platform on the right side of the chamber and go all the way back until you can see the eridian rune on the left side of the chamber through the three rings.