This art is focus on yūrei 幽霊, yūrei simply meaning ghost, and they are a subcategory of yokai. The Pokémon you ca see there are inspired by yūrei from Japanese folklore.
First of all, Typhlosion Hisui places himself here as the leader of the yūrei. In fact, you should know that his signature attack, Infernal Parade , has a reference in the attack's Japanese name. This name is Hyakki yakō 百鬼夜行 (lit. "The night parade of 100 demons") and refers to Japanese folklore, where according to tradition, each year, 100 yōkai storm Japanese cities during the nights of 'summer.
Next, Ghastly is inspired by the yōkai Sōgenbi 叢原火, which is a sort of levitating fireball with a monk's head, named after Sōgen.
Misdreavus is inspired by the yōkai Nukekubi 抜首 (lit. "neck that comes off"), which represents the decapitated human head that levitates and attacks people... When you take a closer look at Misdreavus next time, you will won't be able to forget that it's actually a decapitated head haha! It's obvious in fact, we can clearly see that under the red collar (which reminds us of blood by the way), there is no longer a body.. I told you, yokai are often very creepy !
Sableye is not inspired by a yōkai strictly speaking but it was a special request from MrShadowbox. Since it's also a spectrum, we stay on topic!