Template:New Content That's A Knife is an achievement in Halo: Reach.[1]
It is unknown how the achievement is unlocked, but it seems as though an assassination against an Elite in Firefight or Campaign triggers the unlock.
Sources
Template:New Content That's A Knife is an achievement in Halo: Reach.[1]
It is unknown how the achievement is unlocked, but it seems as though an assassination against an Elite in Firefight or Campaign triggers the unlock.