Type
I think this is a Ghost-type move. You can see parts of the Ghostium Z sigil; namely the upper half-open "eye"-like shape around Mimikyu's face, the wispy "tail" ending between the words 'Ending' and 'Nightmare', and the purple colour. 
(detail of Ghostium Z, Buginium Z, and Darkinium Z Crystals from the Nintendo Australia site).
It is clearly different from a Dark- or Psychic-type Z-Move (the other types of which the name may also imply it to be). http://vignette4.wikia.nocookie.net/es.pokemon/images/1/19/Pulsera_Z_de_TOMY_con_todos_los_cristales_Z.png/revision/latest?cb=20160805012913 (image of the 18 type-specific Crystals. On WikiDex) DecafSnorlax (talk) 14:42, 29 October 2016 (UTC)
- The Japanese toys also have a board with the symbols for all the Z-Crystals. It never occurred to me to pay attention to that (but this is also the only move we can really play this detective work on currently), but Ghost does seem to be the only match for the symbol for Never-Ending Nightmare. Thanks for pointing it out! Tiddlywinks (talk) 15:20, 29 October 2016 (UTC)
- You're welcome! Glad I could help.
