Sinister Strike (commonly abbreviated to SS) is a level 1 rogue ability. You must have a melee weapon equipped to use the ability, and not be disarmed. Sinister Strike can be used on any mob, in any position, in any combat state, and has no refresh rate beyond its energy requirement. You must be facing your target.
This ability is replaced with [Mutilate] for Assassination rogues and [Backstab] for Subtlety rogues.
Patch 9.0.1(2020-10-13): Rank 2 added (Assassination Level 12) with effects: Sinister Strike's energy cost is reduced by 5. Has no actual effect since it's replaced by [Mutilate] on the same level.
Patch 8.0.1(2018-07-17): No longer replaced with [Saber Slash] for Outlaw. Rank 2 added (Outlaw level 12) with effects: Has a 35% chance to hit an additional time, making your next Pistol Shot half cost and double damage.
Patch 7.1.0(2016-10-25): Re-added as Level 1 ability, but eventually replaced for all specializations (Outlaw with [Saber Slash] immediately, Subtlety with [Backstab] immediately, Assassination with [Mutilate] at level 40.