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Missables In AC Brotherhood
Both can be obtained from the beginning of Sequence 2, as soon as Roma loads up press start and select "LEAVE ANIMUS" from the menu.
Check the laptop behind Shaun's computer for the Mailer Daemon achievement, after that go upstairs to exit the Villa and after a short cut-scene of Lucy telling you to bring some gear you will appear outside the Villa. Immediately turn around and just inside the doorway you will find one of the artifacts on your right and you should get the Dust To Dust achievement.