If you want it to be building: Start by looking at the KindomSpecialImprovements.xml. Within the file, check out the Temple of Essence.
If you want it to be a resource (ie: like a gold mine) check out Heavenfall's mod here where he creates a resource that spawns on the map: https://forums.elementalgame.com/398720
Also recommended, which will eventually have tutorials on pretty much anything you'd want to do (Heavenfall's work again): https://forums.elementalgame.com/403584
