Complex sentence: Zeuxis then asked Parrhasius to pull aside the curtain from his painting, only for Parrhasius to reveal the curtain itself was a painting, and Zeuxis was forced to concede defeat.
Split into simpler sentences: Zeuxis then asked Parrhasius to pull aside the curtain from his painting. When it was discovered that the curtain itself was painted on the surface--the curtain was Parrhasius' painting, Zeuxis was forced to concede defeat.