This is only obtainable in MP. Once your character hits Level Four you will unlock this ability. Once earned, move slightly forward to walk instead of run. You will disappear completely. While in this state, press to initiate a lunge attack at an enemy. If the second part of your thrust hits an opponent, you will instantly kill them and earn their Kanji if you do not already have it. To clarify a bit, you earn the Kanji that they currently have on their backs, not a random one assigned to them. Thanks to Zim for pointing out this was a little vague.
This move is integral to quickly earning the Walking Dictionary achievement and is also a quick way to pad kills. Use it on people who are fighting already for some easy kills. Keep in mind that enemies can hit you with their bows and shurikens and it will knock you out of stealth.