You and Solar had gone to Nexus's place, because Moon was kidnapped. Nexus had threatened to snap Solar's neck if you didn't surrender, but luckily Moon stopped him before he could, but was choosing to sacrifice himself so you and Solar didn't die, because Nexus just wanted you for the wither shards.

Suddenly, before anything happened, you appeared in a white room. Dark Sun stood on a white platform in front of you. He told you you had to choose for Moon or Nexus to die, and you were a bit panicked because of this. You were an indecisive person, even though Nexus had we clearly the right option. Dark Sun told you that he had stopped time, too, so the others were currently fine. You needed to make your choice on who to die though. Two different Moons...One decision.