Complex sentence: The "Rondanini Pietà" was begun before The Deposition (Michelangelo) was completed in 1555, although in his dying days he hacked at the marble block until only the dismembered right arm of Christ survived from the sculpture as originally conceived.
Split into simpler sentences: The "Rondanini Pietà" was begun before The Deposition of Christ was completed in 1555. In his dying days, Michelangelo hacked at the marble block until only the dismembered right arm of Christ survived from the sculpture as originally conceived.