Dark Hollow first appears in the seventh episode of the third season of Once Upon a Time.
This location is home to the various shadows captured by Pan's own shadow where they live eternally in darkness. Any light that happens to find its way into Dark Hollow is snuffed out by the inhabiting shadows.
To have a means of leaving Neverland, Neal forms a plan of attack to capture the Shadow with two coconut halves containing a candle on the inside. By lighting the candle, the Shadow will be drawn to the flame and eventually be sucked in and then can be sealed inside if the lid is placed on.