The achievement is unlocked when you backstab your 20th enemy. If you execute a stealth kill in the manner in which it is taught in the prologue, you will have performed a successful backstab. Equip the daggers in either primary or secondary weapon slot. Approach an enemy in the crouched position (dodge-rolls and the dodge-teleport of some higher level destinies performed when in the crouched position retain you in the crouched position). Press the button when you see the prompt (
if the daggers are primary -
if they are secondary). You will know when you have performed a proper backstab when the kill is accompanied by an animated sequence. If in doubt, activate the achievement tracker and make sure it advances on your stealth kills.
This may be performed by any vocation at a low level. The introductory character in the prologue is able to perform a backstab. However, you must inflict enough damage to instantaneously defeat the character. So, increased levels, more powerful daggers, and investment in the finesse Assassin's Art ability all contribute to the lethality of your sneak attacks and increase the enemies that can be backstabbed.
Some enemies are most easily defeated with a backstab. Accordingly, I recommend obtaining this achievement in normal play rather than grinding it out since there are many opportunities when you'll want to backstab as your best tactical option. The below video shows the achievement unlocking in The Natural Order
quest where you must kill ten Jottun. They are powerful, have tough skins, and resist normal attacks. Yet, they are vulnerable to the backstab. Why engage in a prolonged fight when you can get a quick kill? Where you cannot sneak up on them, you can get them to run after you. When they lose interest and turn around, you can sneak behind them using roll-dodge to catch up to them, and backstab them when the prompt appears. I like to equip the daggers as the secondary weapon because then I can perform a backstab whenever the situation presents itself without opening up the inventory menu to change weapons.