In Legion,
Primera arcanista Thalyssra, the leader of the Nightfallen has found a way recently to reactivate the abandoned arcane coils in the ancient tunnel system. With your help, she would be able to redirect the flowing magic to power their base of operation, Shal'Aran.
To unlock Leyline feed quests you need to complete
Energizar Shal'Aran, a part of the
main storyline. After that, the quests for Leyline feeds will become available — and visible by exclamation marks on your minimap even from the surface. You need to find the entrance around the quest markers and enter the underground tunnel to power the actual Leyline feed. The cost is
200 to 250x Maná antiguo per pylon.* Some feeds require you to do additional tasks, like killing certain creatures or to collect special ingredients.