Legitimacy Diplomacy is broken. Total War Pharaoh Dynasties 

The more legitimacy you ask from them the more they like this deal

You can just request Legitimacy from other factions, and it will give you a favorable deal if you request enough. Usually over 15 or so from a faction and the bug is that is counts towards making the deal more favorable the more you request, meaning you can get it for free. The green number under a faction's legitimacy is the amount you can trade with them, so the more they have the more you can get for free, or get them to pay for the privilege.

I just gotten 300 legitimacy with this and the faction even paid me in gold lol. Became Wanax in like next few turns