Mare Nostrum is the 8th expansion for Europa Universalis IV.  The expansion coincides with Patch 1.16.

Expansion features

• Barbary nations can raid for slaves with their ships.
• Steal Maps is a new spy action that lets you conduct a despicable act of thievery and discover all provinces your victim already has discovered in a region.
• Hunting naval mission: Fleets will hunt enemy fleets in a specific region.
• Evasion naval mission: Fleets will evade enemy fleets in a specific region.
• Blockade naval mission: Fleets will blockade enemy ports in a specific region.
• Intercept naval mission: Fleets will seek out and attack enemy fleets near allied coastlines, focusing on troop transports.
• The new diplomatic option Unconditional Surrender forces your enemy to send a peace demand for up to 100 WS within two months, or Call to Peace will be gained at twice the normal rate.
• Having a spy network in another country now gives you passive benefits to siege ability and aggressive expansion incurred against that country.
• Three new espionage actions: Slander Merchants reduces another country's trade power for 5 years, Sabotage Recruitment reduces manpower and sailor recovery speed for 5 years and Corrupt Officials increases corruption.
• The Mare Nostrum DLC also unlocks the Hunt Pirates mission (from the El Dorado DLC).
• Unit View: Added Naval Mission Settings panel that allows you to adjust the behavior of a fleet when on a mission.
• You can now rent out Condottieri to other countries.
• You now press sailors automatically when occupying coastal provinces.
• "Detach Damaged" is changed to "Repair Damaged". Damaged ships will head to port for repairs and then reunite with the fleet they were detached from.
• You can now send a diplomat to conduct counterespionage against a country, increasing the chance of detecting their attempts to build spy networks by 100%.
• Naval leaders are now allowed to be reassigned to and from ships within supply range.
• Promote Mercantilism: Spend diplomatic power to strengthen your mercantilism.
• Added a Timeline feature that allows players to observe how the world has changed throughout the game.
• A leader of a Trade League, can now create a Trading City from one of its provinces. This new nation will have the Trading City republic government and will automatically join the Trade League. (introduced with patch 1.18)