Dark Sister is a Valyrian Steel sword that can be earned by advancing in the quest 'Blood on the Snow'.
It is one handed, swing: 37c, Thrust: 39p, Speed: 106, Reach: 104, Requires strength 12.
As of 2.1 The swords stats are altered, which are as follows
One handed - Swing: 41c - Thrust: 43p - Speed: 115 - Reach 87 - Bonus against shields
You are given the sword by Aemon Targaeryen after defeating the Wildlings under the Wall, together with Jon Snow .