Mark of the Ninja: Remastered

Each level has 9 objectives to do for achievements and skills points to buy abilities.
3 are optional objectives you can read about in pause menu.
3 hidden scrolls which includes a challenge room... And finally..... 3 stars which you obtain by reaching the 3 scores thresholds in the pause menu.

Once you obtain all 3 scores stars on each of the 14 stages in the game this should unlock.