Complex sentence: Mortuary temples were built around pyramids in the Old and Middle Kingdoms, but once the New Kingdom pharaohs began constructing tombs in the Valley of the Kings, they built their mortuary temples separately.
Split into simpler sentences: Mortuary temples were built around pyramids in the Old and Middle Kingdoms. However, once the New Kingdom pharaohs began constructing tombs in the Valley of the Kings, they built their mortuary temples separately.