Complex sentence: It's an allegory about how we're all going to end up and, thus, how transient earthly glory is and was a departure, in tomb architecture, from the usual practice of showing merely an effigy of the person as they were in life.
Split into simpler sentences: It's an allegory about how we're all going to end up and, thus, how transient earthly glory is. It was a departure, in tomb architecture, from the usual practice of showing merely an effigy of the person as they were in life.