Complex sentence: This was the first royal tomb to be discovered that was still largely intact (although tomb robbers had entered it), and was, until the excavation of KV63 on 10 March 2005, considered the last major discovery in the valley.
Split into simpler sentences: This was the first royal tomb to be discovered that was still largely intact, although tomb robbers had entered. And until the excavation of KV63 on 10 March 2005, it considered the last major discovery in the valley.