Apparently it unlocks in the prologue, but it did not do so for me. Any idea how to unlock this ability?

I think you need to find the dead assassin, he´s in the courtyard where the balista is... wher you need to load it and fire it, he´s in the very back of the map

*SPOILERS*

You mean the Assassin ability that increases backstab damge ? If that's the one you have to find the Ass Creed easter egg behind the ballista.

I think I got it where this ballista is, you have to go to the door and find a white dead body ... there it was suddenly unlocked for me.

Ah, okay, I did get this one. There's a lot of confusion around the abilities and how to get them. Some had tapped Assassin and Backstabber as two separate abilities. I also do not have intimidate yet.

Hi SpiralJacobs,

I've not had a chance to play since the 17th but I vaguely remember gaining an Assassin skill which I think increased damage when dealt from behind. Not sure if it's the same skill.

Anyway, I got it soon after firing the ballista to destroy the gate during the prologue. At the back of the courtyard I found a cart with dead bodies in it. That's when the message popped up.

Hope that helps

Edit: must learn to type faster ;)