It’s said that on nights were the sky is clear and the moon clad in darkness,
one can hear bells rings echoing through the air even though there is no steeple around,
they whisper the same litany over and over again :
Ph’shagg ph’syha’h cth’n tharanak Gr’ghr ch’shugg.
-A bat pony legend.

When Grogar invade Equestria and unleash his undead armies on the land, Celestia and Luna go inside Tambelon to stop him, a group of six royal guards follow them in hope to assist them. They don't know that by doing so, they entered a world filled with horrors from beyond this universe..