This achievement is for completing all 10 of the Memory Sequences. Here are Shadow Gold walkthroughs for each sequence. Credit to Centerstrain01 for the videos Memory Sequence 1- The Assassin's HeartMemory Sequence 2 - The Absent HandlerMemory Sequence 3 - The Quest BeginsMemory Sequence 4 - The Secret BelowMemory Sequence 5 - The Enemy RevealedMemory Sequence 6 - The Silent AssistMemory Sequence 7 - What Lies BeneathMemory Sequence 8 - The Thief WithinMemory Sequence 9 - The EscapeMemory Sequence 10 - The Rescue
The achievement tracker for this achievement appears to be broken. Sometimes I got progress at the end of a Memory Sequence and other times it increaed after replaying a Memory Sequence. However even though it was showing 60% when I started Memory Sequence 10, it unlocked at the end of the game, so do not worry.
If you want to get all the achievements in this game in just two playthroughs I recommend you do not kill any additional enemies on this playthrough to get: