Memory powder

 is an item that's used in creation of during a Game of Crone questline. After unlocked the quest and talked with the witch Clotho, she gives you three for free. If you need more, she provides additional ones for each. Be aware that, while it appears to be an alchemy reagent, and is used in a recipe at an alchemy workbench, it can't be used in any other capacity - such as, as a common alchemical ingredient.