The Longline is a discovery in Skies of Arcadia Legends. It is found in the Looper territory, which is the triangularly shaped section in the northwest part of the map. This discovery is fairly easy to find; simply travel west from the Ruins of Ice in upper sky until you are atop the Dark Rift and you should find it. Note that it does move a little bit, so if it isn't there, give it a few minutes to show up.