Updated Jan-10-12
A Dark Eldar Scourge is a particularly rich and daring indivual who has submitted himself to a haemonculus's attention - having his bones hollowed out, powerful wings grafted to his frame and new bands of muscle and adrenaline dispensers added to his torso so that he is capable of true flight. Unlike other armies, who have assault-oriented jump-troops, Scourges carry heavy weapons into battle, often employed for hit-and-run tactics against vehicles and heavy infantry. A scourge squad leader is known as a Solarite.
Sourges are awesome. I don't care what anyone else says!