The Black Knight is an enemy that will spawn at random during matches. He will take over all of the rally points, turning them yellow and begin to attack anything on the field. To help defend against him, you should try to acquire the "White Knight Shield" before your opponents to give you immunity to his attacks.
To banish the Black Knight, you need to take over all of the rally points on the field. To do this you need to move your Rally Troop over one of the rally points and your troops will begin turning it back to your color. Take over all of the rally points that the Black Knight has turned yellow and upon taking the final rally point the Black Knight will be banished. Be careful not to rid the field of the Black Knight through damage as this does not count as a 'Banish' and thus does not count towards this achievement.
You can unlock this during a quick match and have a second controller connected to make this achievement extremely easy.
Banish 50 of these Black Knights and the achievement is yours.