if the shadow squad represents the stealth and tactics of the alpha legion 

while terror squad represents fear and terror of the night lords 

the executioners are considered the most deadly of them 

all instead of having soldiers from the legion drawn

they piloted powerful mechs designed to hunt their targets 

especially traitors and those that support the black arms 

when they come for  you better pray they kill you quickly as they show no mercy 

to the executioners the punishment for betrayal is death