Direct sequel to Devourer, which is required reading for this.
Character Tags added as needed.

Ever since its failure to make any lasting changes during its stay in Canterlot, the Devourer hasn't felt right: everything it does feels hollow. It can't even enjoy things it used to love doing, such as digging up gems for the dear queen.

Tired of these feelings of emptiness and inadequacy, it approaches its queen with a request to do something it has not done in many hundreds of years.
It wishes to go on a journey to take vengeance on a select few of the ponies it blames for what happened, to travel to the Crystal Kingdom alone where they now reside and deliver its justice.
It wishes for the blood of Shining Armor and Cadence.