Use: Permanently teaches you this glyph. Curse of Exhaustion now reduces the targets movement speed by 30%, lasts half as long and has a 10 second cooldown.
Many NPCs have a faster baseline running speed than players, making this curse more useful in PvP than in PvE.
CoEx can actually be very useful in catching those enemy players who are trying to escape from you and/or your allies, especially if they're carrying the flag in a capture-the-flag game.
Patch changes
Patch 4.0.6 (8-Feb-2011): Curse of Exhaustion now reduces movement speed by 30%, down from 50%. Its range has been increased to 40 yards, up from 30.
/Patch 4.0.1 (12-Oct-2010): Now slows movement by 50% and lasts 30 seconds (Previously 30% for 12 seconds).
/Patch 2.0.1 (05-Dec-2006): Changed from reducing speed by 10% and requiring 4 more points in Improved Curse of Exhaustion to reduce it by another 5% per point. It used to require five talent points to reduce the target's movement speed by 30%.