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Compare the Japanese names of Ghost-type moves to the Japanese names of Dark-types and you'll find that there are way more Ghost type moves associated with darkness and shadows. Lunala's signature move (Shadow Ray) matches most damaging Ghost move names. Also Dusk Stone (Darkness Stone) has more Ghost moves.
>inb4 but muh Black Hole Eclipse
Japanese name for the Ghost Z-Move is Invitation to the Endless NIGHT.