The Main Six have just defeated The Storm King, thus saving Equestria. They then celebrate their latest achievement with their friends and loved ones. However, during said celebrations, an eldritch horror of unknown origins rips a hole into the very fabric of reality; using the moon as a doorway to another world.

[Story continues in Terraria: Hardmode.]

Warning: Comment section may contain spoilers.

Foreknowledge of "Terraria", isn't entirely necessary, but it does help nonetheless.