When wielded by an Eldar user with a Psy Rating, a Witchblade grants its user the Unnatural Strength (x3) Trait for the purposes of dealing Damage, and gains Penetration equal to the wielder's Psy Rating. It does not count as a force weapon. Human psykers are unable to make use of these abilities. The Warhammer 40K table tob witchblade triples its users strength, in the Black Crusade book you can find a witchblade being used by a harlequin, if you break it down you will find it uses the same rules as this one.
Due to their nature, Witchblades are never available for normal purchase, and must be acquired through the black market or from the Eldar themselves on a case-by-case basis.