The official Total War wiki has recently been updated, showing off some details and story of the Egyptian faction and its role in Total War: Rome 2.

"A general under Alexander, Ptolemy was appointed to govern Egypt after his king's sudden demise. With Alexander's empire fragmenting, Ptolemy declared his independence, securing Egypt against his power-grabbing rival generals during the Wars of the Successors (322 – 275BC)."

For more details on the faction, the full page can be viewed here.