Complex sentence: But to their dismay, a tribe known to be the Hyksos surprised the Egyptians they marched into Egypt during the 15th Dynasty in the Second Intermediate Period with chariots and took the people of Egypt by surprise.
Split into simpler sentences: But to their dismay, a tribe known to be the Hyksos surprised the Egyptians. They marched into Egypt during the 15th Dynasty in the Second Intermediate Period with chariots and took the people of Egypt by surprise.