The dagger one is after the bow one. Ur thinking of the one-handed perk, u can use sword, axe, even a mace. U will get more damage (since they do more damage than a dagger). The dagger perk's name is "Assassin's Blade" or something.
http://www.uesp.net/wiki/Skyrim:Sneak
Backstab 00058210 30 Sneak Stealth Sneak attacks with one-handed weapons now do six times damage. Deadly Aim 001036f0 40 Sneak Backstab Sneak attacks with bows now do three times damage. Assassin's Blade 00058211 50 Sneak Deadly Aim Sneak attacks with daggers now do a total of fifteen times normal damage.





