I took a quick look at the most recent Assassin's Creed game. Turns out that all you need to do to become a successful assassin is to learn how to sit on a bench...
It's a bit silly that you have to purchase the "skill" of sitting on benches, but then again - I guess it's understandable if French Ezio doesn't want to sit on public benches... Who knows what stuff is on them :/
Game: Assassin's Creed Unity
Platform: PS4
The song at the end is 'Stabby Stabby' by Dexter Manning from Sanity Not Included (props to anyone who got that reference without checking the description first)