Follow de Molay to find the Templar artifacts, then kill the assassin.
Assassin’s Creed: Unity guide – Prologue: Kill The Assassin
The prologue mission of Assassin’s Creed: Unity acts as the first of several tutorials to come, so feel free to give yourself a little time to become familiar with the new control scheme as you complete a series of pretty simple objectives. There are no additional challenges to shoot for here.
Follow de Molay
All you have to do here is what it says on the tin: follow de Molay. You’ll be prompted to run, so now’s the time to familiarise yourself with how to do that, but this won’t be a long chase; you’ll hit a cutscene and be given a new objective.
Find the Templar Artifacts
You’ve barely been playing for long enough for the cat to lick its ear and already you’re in a brawl; not the most auspicious of beginnings to your stealth career, but you’ll need to know how to get out of trouble when you inevitably get into it, so no complaints.
You’re learning how to attack, dodge and parry in this battle, so obey the prompts in order to proceed. Once you’ve managed that, you don’t necessarily need to kill all your opponents; just make a break for it when you can get clear. You can skip the battle here altogether if you’re a canny runner, actually.
Keep moving towards the fortress to trigger another cutscene denying you any possibility of a quick entry across a drawbridge. Not to worry; turn left once the cutscene ends to spot a ladder leading to a window. Climb up for your first experience entering one of the many, many explorable interiors in Unity. There’s little to do here, so just pass out the opposite side.
Once outside again, look to the left for a ladder on some scaffolding. Clamber over it and continue upwards, obeying the tutorial prompts to learn how to make your way up a wall and into another room. From here, take the spiral staircase downwards to run into another cutscene and a new objective.
Kill the Assassin
This is an extended chase as well as a battle, so use everything you’ve learned so far along with a series of new moves taught as you go to repeatedly catch and batter the assassin.
You’ll need to go through several rounds of this before the enemy goes down, but it’s not too tricky. Once he’s dead you’ll view another cutscene.