On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled The Chosen One. You can start this quest in the Sanctuary location after completing the main mission named Where Angels Fear to Tread.
2) ECHO Recorder 1
4) ECHO Recorder 2
6) ECHO Recorder 3
Head to Marcus' shop  to receive a mission of finding his debtor. Use Fast Travel and move to Sawtooth Cauldron. If you haven't yet activated the device there, travel to The Dust. From there head to Eridium Blight through the passage in the west. In Eridium Blight, in the north-east part of the map, you will find a passage to Sawtooth Cauldron. The first ECHO Recorder can be found in the camp in the upper part of the map .
In order to save time on unnecessary travel, jump into the camp below (screenshot) in the spot marked as . Below, on one of the chests you will find another ECHO Recorder .
Inside the container marked as  you will find the corpse of Kai. If you want to receive additional experience points for finding three ECHO Recorders, you will firstly have to head to the spot marked as  for the last recording. After finding the body, return to Marcus  to end the mission. the way back to the Fast Travel lead through passages  and , underground or along a road up . That way you will find yourself in the camp where you collected the first recording .