Total War: Three Kingdoms - Cover And Underground Network Guide
In Total War: Three Kingdoms, you can send spies to infiltrate enemy factions, and undermine their position, or even cause proxy wars.
While it takes a few turns for your spy to infiltrate the faction, once they do they will begin earning a currency called Cover. The amount of Cover you have will increase each turn, all while building up a second currency called Undercover Network. Both of these currencies can be used to manipulate the enemy faction from within.
Undercover Network is an indicator of how deep your agents have infiltrated the target faction. When used together, these currencies allow you to influence your enemies for your gain.
How Spy Currency Works
After your spies have infiltrated the other Faction, they will take up a position as a Court Noble. This will give them access to certain actions such as pursuing military appointments, discrediting other characters, interfering with the faction's efforts, and even discrediting the faction as a whole.
To carry out these actions, you will need to be able to meet their cost in both Covers and Underground Network. It is entirely possible for a spy to be promoted within the enemy faction until they are a member of the ruling family. Each new rank they attain within the Faction will open up new actions that they can take to undermine it.
You do need to be cautious though, there can be a risk that your spy will get so involved with the other faction that they start to value its goals over your own, and there is a risk of defection.
From the Undercover Network screen, you can check to see if your spy is close to defection, and can either disown them, pre-empting any damage they might do, or recall them back to your own Court.