The teleport beacon starts at level one. By putting a Magic in it, it upgrades the teleport beacon another level permanently. Maximum level is 2. Magic is used up permanently in the process. The higher level the beacon, the longer the maximum range of teleportation.
Use this ability on an allied troll. The troll is teleported to the same position on the opposite side of the tower. Maximum Range is determined by the level of the tower. See diagram for a simple example on functionality.