The Sky Darkeners are mindless constructs with one purpose: KILL! Specifically, they are aiming to kill any man or woman they see fleeing Havenshire for the safety of New Avalon. We certainly can't allow those cowards to make it to safety and then take up arms against us!
There is one slight problem. They use saronite arrows for maximum range and damage. Saronite, however, is a metal only found in Northrend, so supplies are limited. Retrieve those arrows for me and I will reward you handsomely.
A reference to the film 300, a line from Gerard Butler: "Spartans! Ready your breakfast and eat hearty...For tonight, we dine in hell!"
The "Sky Darkeners" may also a reference to 300: A Persian emissary, upon losing his arm to Spartan warrior Stelios (Michael Fassbender), threatens the Spartans with annihilation and boasts that "our arrows will blot out the sun" (to which Stelios replies "Then we will fight in the shade").