You used an enemy lifted with Pull as a shield

You delivered (or attempted) the killing blow on Corvo

You performed a drop assassination from atop the Empress statue in The Knife of Dunwall

You caused a death using an arc mine and a rat

You purchased all of the Favors in The Knife of Dunwall

Completed Episode 5: "No Going Back"

Reached the terminus.

Reached the other side.

Took a breather.

Completed Episode 4: "Amid the Ruins"

Survived the attack.

Found the Observation Deck.