Dominus (plural, domini) is a powerful entity that governs a domain.   The relation between domains and their domini isn't fixed, and neither is the nature of domini, ranging from sentient entities to landmarks that are fountains of mana. Given the subtlety of these relations, Domini are divided into four categories for ease of study: Lords, Locii, Avatars and True Dominus.  

Domini Types

Lords

Lords are the most common type of Dominus. They ascend to their position by conquest, birthright, ritual inheritance or by favor of the Locus (explained further). Though usually powerful entities to begin with, their power as Lords comes from their condition as vessels for the power of the Domain.

Locii

Locii (singular, locus) are the nexus of power of a domain, usually a natural formation like a waterfall, a lake, a mountain or a great tree. They might exhibit behavior akin to sentience in a way that sentient beings can study their ways by observation, buy are mostly the heart of the Domain. It has been theorized that an entire Domain might be part of the Locus, but this is suboptimal from the perspective of a conquest and they would probably be considered Unclaimed Lands.

Avatars

Avatars are extremely powerful entities borne from and by the domain itself - like a sentient Locus. The circumstances that lead to the creation of an avatar are unknown and their appearence varies wildly according to the Rubedo of the Domain. They are sentient entities of great power, some argue close to Gods when inside their Domains, more so than Lords. Their source of power is also a shackle, as avatars cannot stray far from their Domains lest both slowly perish - though depending on their power, this time window might be centuries long. Lesser Avatars or Keepers are a kind of entity created by Loci in a similar fashion, although much weaker.

True Domini

True Dominus are the rarest kind of domini. They are godly beings that shape their own domains through sheer power of creation. Unlike true Gods, true dominus do not (usually) have the power to create a new plane on a whim, but they have the power to claim unclaimed lands such as Crossings in the Otherworld and shape them according to their devices. Sometimes they can even overcome the power of loci, avatars and other lords to claim a domain as their own, usually to catastrophic consequences.  

The Lordly Bond

The Dominus and the Domain share a lasting magical bond. When sentient, the dominus can tap into this bond for various effects, according to the depth of the bond and the dominus' power.   For instance, Lords can usually sense magically significant events that happen in their domain. Some can even sense when creatures cross into their boundaries. They can also tap into the domain's mana, giving them deity-like powers when within their domains, and sometimes even outside of them. They can direct the flow of mana to certain regions, speeding up the growth of vegetation, enchanting creatures and creating wellsprings. Powerful Lords can even create new lifeforms (often by manipulating existing ones) and shape landscapes.   This bond works both ways. The mana of the domain also inevitably changes the nature of its dominus, altering its behaviour and even its magical signature irreverseably. Domini tend to embody the will of the domain as much as they use the bond to change its form.