Notes:

dA source
Naver source

The artist took inspiration from when Sombra fell into an icy hole and got frozen like this during the Season 3 opener.

The artist notes that she wanted to put Mane 6 at the beginning. However, according to the artist, the horror movie cliche dictates that those who actually summon evil spirits/antagonists are often dumb characters, and therefore decided against putting Mane 6 and was going to put notable background characters like Lyra or Bon Bon. This idea, again, would provide much disservice to those characters, so she just made up some background OC ponies (OC background ponies?)