Tree: Affliction
Curse of Exhaustion is a Warlock spell attained by putting points into the Affliction talent tree. It has a prerequisite of 
[Amplify Curse]. One point of Curse of Exhaustion gives you the spell, reducing target's movement speed to 90%. Four more points reduce it by another 5% per point, so with five points the target's movement speed can be reduced to 70%.
On its own, this talent reduces the target's movement speed by 10%. It's important to note that even at 90%, NPCs still move much faster than you so this spell will not help you escape. With all 4 ranks of 
[Improved Curse of Exhaustion], the reduction becomes 30%, and NPCs move slightly slower than you. When you add [Amplify Curse], you get a 12 second 50% snare every 3 minutes. This talent is more effective against other players than NPCs since they start out at the same movement rate as you. Some Warlocks swear by this talent, some just swear at it.