A tether is an effect that creates a link between the source and the target(s) whose power can be dependent on the effective range and/or time linked. The link is broken beyond maximum range and if either the source or the target(s) dies. Some hostile tethers reveal the target, and tethers will also reveal the source if the target is a champion. Tethers persist through untargetability. Spell shields can prevent the application of hostile tethers but not their after effects (this varies). Dispel effects and the transformation into the zombie state will always remove tethers. The transition into the can break any existing tethers on differing targets.
Types of Tethers
A hostile tether is a link between a source and an enemy.
A friendly tether links between a source and an ally. It is reapplied instantly if the two become in tether range of each other again.