At the end of the third sequence you will unlock the Wolf Cloak. This will allow you to go invisible. Equip it and hit to activate. Equip the hidden blade then walk behind people and stab them. You can not sprint and the longer you are cloaked the more health you lose. So it is advised to only kill one or two enemies at a time and then hide to regain your health. This is cumulative throughout your playthrough.
For some players this achievement requires more than 25 cloaked kills. If that is the case, just keep killing people and it will unlock sooner or later.