Diplomacy in Age of Wonders 4 is governed by the War Justification system, where your actions are measured through Grievances. These represent historical wrongs or territorial disputes that provide justification for war; declaring war without sufficient Grievances incurs heavy Imperium penalties. You can manage these tensions through Pronouncements, such as declaring a Godir your Rival to generate monthly Mana or forging a Friendship to build trust.
To influence independent Free Cities, you must utilize Whispering Stones. Assigning these magical artifacts to a city increases your Allegiance over time, eventually allowing you to negotiate Pacts or fully vassalize the faction. Whether through trade, threats, or strategic alliances, mastering these core diplomatic mechanics is essential for maintaining your empire's stability and navigating the complex relationships of the Astral Sea.