Othercide

This will likely be one of your final two achievements. You can activate a Remembrance during a Recollection (run) provided you have enough unspent shards.

There is a large list available in your Codex, inside your Inner Void on the Chrono Map screen. When you start a new Recollection (by choice or having been forced due to failure of the previous one) the game will ask you which Remembrances you want to activate. Simply check them all, oddly you don’t need to hit to Validate, the achievement pops as soon as they’re all checked. You can then uncheck any you don’t want to apply to your next Recollection and Validate.

You will need a total of 2500 Shards to activate them all. This isn’t as hard as it sounds as Shards, once awarded, are added as a cumulative total (e.g. if you earn 700 Shards during your first Recollection and only 100 during your second, you’ll have a total of 800 to spend for your third). Every mission is worth 25 Shards and clearing a boss is worth 50 shards (thank you ShadowEchelon91) so they add up really fast.

Of course to activate all Remembrances you’ll need to earn them all. Click on the spoiler for any final Remembrances you might be missing.