Assassin's Creed II

You can unlock this as early as DNA Sequence 11. After completing the memory ‘Practice Makes Perfect’ in DNA Sequence 3, you are able to place the feathers you’ve collected in a chest on your mother’s dresser in Monteriggioni. Once you have placed all 100 feathers in the chest, there will be a short cutscene with Ezio hugging to his mother and the Auditore cape will be unlocked.

Note: When you’ve deposited 50 feathers, you will unlock the ‘Condottiero War Hammer’ to purchase at Monteriggioni’s blacksmith for 35,000f (less if you’ve upgraded the Blacksmith).

Once you have control again, to wear the cape, pause your game and navigate to Inventory > Outfits > Auditore Cape and press to equip it. The cape gives you instant Notoriety in any city you visit and you can’t lower it by tearing down posters or bribing Heralds until you switch to another cape.

Now, to unlock this achievement, you need to fast travel to every city (located by the icon on the map). Go to the fast travel station outside the gates of Monteriggioni and select another city. When you appear in that city, just repeat the process. Once you have selected all four (Tuscany, Forli, Venice and Florence [go here last]), you must also visit the Apennine Mountain region where you met Leonardo and had that carriage chase before arriving in Forli during DNA Sequence 6. Fast travel to the ‘San Marco - East Gate’ in Florence (since that is the closest station to the Mountains), walk out of the gates and get on a horse. Take the northernmost path to travel to the mountains. The achievement will unlock as the game is loading.

Refer to “In Memory of Petruccio” for a video showing the locations of all 100 feathers.

Thanks to ElectronicElation for the video: