Complex sentence: The only Reaper to be witnessed by any living intelligent being after the Protheans extinction was a dreadnought designated Sovereign, several times the size of any known vessel--even dwarfing the massive asari flagship, Destiny Ascension.
Split into simpler sentences: The only Reaper to be witnessed by any living intelligent being after the Protheans became extinct was designated Sovereign. It was a colossal dreadnought, several times the size of any known vessel--even dwarfing the massive asari flagship, Destiny Ascension.