Eldar

Across the galaxy can be found the scattered remnants of the ancient Eldar race, most of them drifting across the voids between stars on gigantic vessels they call craftworlds. The warriors of the craftworlds are varied and powerful, each with their own particular prowess and skills.

The deadliest Eldar fighters are the Aspect Warriors, whose ritualistic combat styles mean they excel at one particular form of fighting, such as close combat with powered weapons, long range support with heavy firepower, or tactical versatility.

Combined with the strange and sophisticated arms and war machines of the Eldar race, their warriors are frightening opponents able to defeat foes who would otherwise outmatch them in numbers or firepower.

-from the Warhammer 40,000 rule book

The Eldar are versatile and deadly, and definitely not to be trusted. They have an odd combination of strange weapons and what-have-you that can take you by surprise and devastate your army before you know what hit you.