The Defiled Reins awards the user with the Grove Defiler moose mount, and in order to get this you need to complete the Glory of the Legion Raider achievement. This is a meta-achievement in which you have to complete a total of 19 raid achievements with your raid group. The achievements require a certain strategy to complete, and is spread across The Nighthold and The Emerald Nightmare raids.
This means that you need a skilled group of players to defeat the raid bosses in a certain way. The achievements can only be gotten on normal difficulty or higher, so you cannot complete these in the LFR.
One you manage to complete every single achievement, you will be awarded the meta-achievement, and the Defiled Reins will be in your mail box!