The Duality

This custom challenge forces you to stay underground (below y:60) during nighttime, and above ground (above y:50) during daytime. The area between those y-levels is always safe. To break this curse, you have to defeat 2 custom bosses, the Sun Emperor and the Void King. They may look familiar, but do not underestimate them, they are a force to be reckoned with.

Walkthrough (Mild Spoilers)

Sun Emperor

To summon the Sun Emperor, you need to craft an arrow of light using and Arrow and surrounding it with Sun Essence, which is dropped twice from iron golems. You then need to shoot it upwards at daytime and the Sun Emperor will appear.

Void King

To summon the Void King, you need to craft a diamond block, which now only consists of 4 diamonds. When you break it, you will get a Void Crystal, which will summon the Void King when thrown onto bedrock.