My impression of Memoria was that it was similar to FFVI's interpretation of Magicite, in that it stores a primal's power on their defeat. For practical purposes it's probably related to Amaurot's concept crystals that were used to store Creations, except that these leak out aspected aether that can corrupt the wielder.

The main difficulty with this sort of a technique lies in drawing a distinction between it and other forms of magic. If I use auricite to bring forth a primal, is that not summoning? If I'm using crystals to form and break covenants, is that not void magic? A shapeshifter concept would be great though, especially one that involves partial shapeshifting. I really like what they did with Barbariccia's ability to partially alter herself with giant punch attacks.