First Strike I rewards opening a fight on your terms with a big hit before the enemy is ready. Two points at level 2.
First Strike I sits at position 14 of 19 in the Stealth tree. Unlocking First Strike I takes 2 skill points. You need level 2 before you can buy it. Jacob and Evie each have their own skill tree, so you buy First Strike I once for each twin.
Air Assassination
Drop onto a target from above and kill him in a high-profile strike.
Assassin Steps
Greatly reduces fall damage, silences your landings and makes your movement much quieter.
Chameleon
In Sneak Mode, Evie becomes invisible while standing still; only enemies very close by can spot her.
Double Assassination
Kill two nearby unaware enemies at the same time.