V Jump has stated that this month’s chapter was the conclusion of the Magistus arc, and the 3rd story will be the “Magical War”, i.e. the conflict between Endymion and the Prophecy monsters described in Master Guide 4.

They tease that the child of Artemis/Selene/Reflection will be the key to the plot. For those who haven’t been following the manga, this kid was introduced in the first chapter of the Magistus arc, which has been framed as the mother telling them the story of how she met Endymion… 100 years earlier.

Since part 3 is a direct sequel to part 2, expect the Witchcrafters and the Invoked to pop up as well.