I have a story set in an alternate version of 1998 USA where nearly every single mythical humanoid humanity has ever believed in actually does exist (or used to before going extinct) and lives in secret among humanity. They are collectively known as "immortals", due to their eternal youth and immunity to disease. While individual humans do know that immortals exist, broader society must not be allowed to learn of, and believe in, their existence. This "Veil of Ignorance", as it's called, is enforced by a group known as the Veilkeepers, members of an immortal species with great power over the human memory, who patch up secrecy leaks as they occur and punish the offenders with death.
Beyond this, however, the Veilkeepers have very few rules, and every single one of the few rules they do have is focused on making sure immortals aren't discovered. They don't actually dictate or enforce any rules against crimes like theft or murder, especially not against other immortals. As a result, immortals have had to take it upon themselves to enforce order, and have formed assorted "clans" who control varying amounts of territory and have their own laws they expect immortals who live in their territory to follow.
There is a problem, however: the mechanics of it aren't fully understood, but the process of using magic seems to produce something that for the purposes of this question I shall call "felltale radiation". And during every full moon, at the instant when the moon reaches peak fullness, time stops for 24 hours for every non-magical creature, and excess felltale radiation seems to produce dangerous monsters known as felltale during this period, which hunt down and kill any immortals they can. If the felltale get too numerous or too strong, there will almost assuredly be casualties among the immortals during this "moontime". Felltale radiation can only be dissipated by the passive presence of humans within approximately 10 miles, and as a result, it's been worked out that the immortal population needs to stay beneath roughly 1 in 1,000 humans in order to avoid having to fight the felltale on a monthly basis.
There are two major takeaways from this:
1: Clans need to expand their territory if they want to be able to survive having more members. Otherwise, they'll be subject to violent monthly culls. As a result, they're frequently incentivized to have behind-the-scenes "wars" with neighboring clans, usually during the full moon when they don't have to worry about humans watching.
2: Clans trying to control territory have an inherent homefield disadvantage. Visitors can bring in much larger numbers of immortals than a location can normally withstand in order to take out the immortals living there, whereas the people on the defensive have to make sure they'll be able to survive within the territory they control once the full moon comes, and thus can't do much to fortify its defenses with long-term reinforcements.
And this second issue only gets more severe the less densely populated the area is. Places that are more densely populated by humans, like cities for example, allow a larger amount of continuous immortal presence, and thus are more easily defensible by the clans that inhabit them because amassing the numbers necessary to take over is a much bigger undertaking. Less densely populated places, places that perhaps can only support an immortal or two per square mile, would be things that any clan that has control of so much as a single city could amass the numbers to conquer with ease.
This makes me suspect that clan territory control would be almost exclusively focused on cities, while more suburban or rural areas would largely be volatile frontier that no organized clan would try to secure an active presence in. But I want to run this past here first to make sure I'm not missing anything.
Given the system and limitations outlined above, would immortal clans have any methods of holding on to less densely-populated territory?