the harlequins are a race almost as ancient as the necrons who are out to avenge their creators, the old ones, by killing the necrons to extinction.
A (very) small subsect of the Aeldari as a whole, the Harlequins are an elite close range force that has a strong melee punch as well as lots of ways to control the flow of battle.
Harley Quinn's abilities include expert gymnastic skills, proficiency in weapons and hand-to-hand combat, complete unpredictability, immunity to toxins, and enhanced strength, agility, and durability.
Harlequins are an army that moves fast, hits hard, has 4+ invulnerable saves across the board (without having to be Custodes), and has an irritating amount of luck.