Complex sentence: MAT believed the object harmless and left it with them, but it began to flash and move on its own, breaking out of the container, quickly growing to giant size and hatching like an egg, Sartan emerging.
Split into simpler sentences: MAT believed the object harmless and left it with them. However, it began to flash and move on its own, breaking out of the container, quickly growing to giant size with Sartan emerging.