Stalking Hunter (3.5e Alternate Class Feature)
From D&D Wiki
Some Rangers forsake their styles of combat training in favor of devastating back stabs.
Level: 2, 6, 11
Replaces: Combat Style, Improved Combat Style, Combat Style Mastery
Benefit: You gain the sneak attack ability. At 2nd level, your sneak attack damage equals +1d6. At every four levels thereafter, this damage increases by +1d6. (+2d6 at 6th level, +3d6 at 10th level, +4d6 at 14th level, ect...)