Except for hellions, most immortals tend not to have large families or large populations. Because of their long lives, immortals tend to reproduce excruciatingly slowly. And since most of their kind were confined to the netherworld after the Gods' War, it will take an even longer time to expand their population. Between slow breeding, interbreeding with humans, recessive abilities, and powers skipping generations, it's not uncommon for certain types of immortals to be the only one of their kind they know.
Since they lack family ties of their own for community and protection, most immortals form alliances among themselves in the form of clans. Instead of biological and marital relationships, the immortal clans are based on loyalty oaths. Just like real families, each immortal clan is different. In Blackmire, immortal clans are more like street gangs or mafia dynasties. Though their individual schemes vary, most clans operate protection rackets in their territory, demanding "tribute" from the locals and even lower ranking clans.
Vastienne's clan is the most powerful and occupies the highest levels of the city. She runs her clan like a royal court with strict hierarchy and elaborate rules of etiquette. While some clans use tattoos to denote membership, Vastienne's clan members wear elaborate bespoke jewelry bearing the clan symbols.
The Leviathan Street clan's territory covers the River District, the lowest levels of the city, known locally as Downside. They're more like a family. A violent, but extremely loyal, family. Their members are tattooed with an elaborate, moving leviathan curling around the forearm. The bigger the leviathan, the higher the member's rank within the clan.