This achievement is for the 'good' ending, which can be completed by collecting at least 20 of the Memora collectibles before entering the Vault of Nyx.
*** Spoiler - click to reveal ***After destroying the Animus in the Vault of Nyx, you'll be prompted to press 
on a tree stump. In the good ending, you'll place Cabrius' death mask on the stump
There are multiple ways to find Memora - Red Memora are all over the place and can be collected once. White-glowing Memora are story/quest related, and more are spawned when accepting a Memora quest. It's quite easy to find 20 just through exploration (I ended up with 29 at the end of the game), but if you can't find enough, doing faction quests can help you get the needed amount.