In the Europa Universalis IV: Leviathan expansion, you get new tools to centralize and accumulate power for your country. New diplomatic actions give you the means to go up against mightier counterparts and new economic tools will allow you to establish rich vassal empires that draw wealth and development to your monumental capital city.
Leviathan also brings new improvements to other core areas of the game, such as colonies and regencies.
Ask for Favors: Curry favors with other nations and use them to advance diplomatic goals.
New Regency Options: Regency councils represent the interests of the most powerful estate, and can be extended to delay the ascension of an unfit monarch.
Specialized Colonial Nations: Let your colonies focus on military help, trade power or self-government.
Concentrate Development: You can now steal development from your vassals or territorial possessions to enhance the power of your capital city.
Pillage Capital: Loot an enemy capital as a condition of peace, hauling development back to the capital metropolis.
Expand Infrastructure: Provinces can expand their capacity to build new structures and manufactories, allowing smaller nations to create centers of wealth for a modest cost.
Centralize State: Reduce the cost of government by spending unused Reform points.
Totemism: Nations that follow the Totemist religion can revere a pantheon of past leaders, earning bonuses that reflect the skills they had in life.
Monuments: Expand and establish great projects, adding new bonuses to your empire.
New Unit Models: New army sprites for Southeast Asian nations, including Indochinese, Indonesian and Polynesian nations.
And more: Including the ability to carpet siege enemy provinces, draft transports as a plutocratic nation, watch heirs gain legitimacy as they wait to inherit the throne and other changes.