In Tlessa when a kingdom is abandoned or neglected for longer then 90 days, the kingdom will fall into ruin. During this time the kingdom will turn from blue to yellow, or red to yellow.
A yellow kingdom is up for grabs, any one, any where can purchase the kingdom or wage war on the kingdom and take it for their own.
As you can see to the right or the bottom for mobile, there are a series of yellow kingdoms. Each one of these is a NPC kingdom, probably abandoned by their owner or neglected over time and fallen into ruin.
When you click on one, or if you are on mobile and visit a NPC kingdom and click View Location Details, you will see some details about the kingdom, as you can see to the right or bottom for mobile.
Here you can purchase the kingdom.
The same rules for purchasing an NPC kingdom apply to settling a kingdom, its 10,000 Gold * the amount of kingdoms you currently own across all planes.
If you own no kingdoms and you purchase a NPC kingdom, it will be free, much like settling a kingdom.
You can also see that you have the option to attack the NPC kingdom and take it by force.
If the kingdom was abandoned due to neglect, it could still retain all it’s walls, building levels and units. Those units and the associated defense bonuses will still defend and fight for the kingdom when you attack it, best to save your units and just purchase the kingdom.