Exhaust

Exhaust is a 'targeted' Summoner spell, which slows movement speed and damage dealt by 30% and reduces attack speed by 50%.

Strategy
Exhaust is an extremely powerful tool for disabling champions who mainly autoattack, as well as any champion that can deal a high amount of damage. When your team is chasing an enemy and he is getting away, using Exhaust on him will help your team gain the upper hand. Using it against enemies when escaping can help you as their movement speed will be decreased.