Soulmates (The Mark of A Beloved)

Note: Images from Google, to explain things easier.

Soulmate marks are typically formed around the age of 16-18, depending on the creature/species. Each species seem to have a different soulmate mark. For Sirens, Mermaids, Ravenstags and Vampires, their marks appear as writings on their arm, neck or chest. The words are random, usually some word that either embodies your mate or a word that was uttered the moment the pair knew they were soulmates. For werewolves, they know their mates by scent and they have periods of heats or mating season. A quirk that is often mocked at by the vampires, for being barbaric or 'mutts'.  

Humans and Dream Eaters (Cthulhu descendants) don't have any soulmate markings, but they do have soulmates. For the latter, their soulmate is usually their kin and is more a familial bond than love interest.

For witches, their soulmate marks are drawings that depicts their soulmate. Witches have an interesting mating process. They become one body with their soulmate, if their soulmate fuse with them willingly, you can see the other mates form as their partner's shadow, happy and calm. However, if they are paired by force, the shadow will seem erratic, chained up, or crying. Witches also often search their soulmate more zealously than others because they need their partners life force to survive, as the more spells a witch casts the more it drains their life force. So they tend to care for their soulmates very much.

It's the complete opposite of vampires who fear/are disgusted by their soulmates. This is because the witches curse back from the great war, vamps now have mortal/short aged mates. And their mate is never their own kind. 

For a vamp to fully bond with their soulmate, they need to drink the others blood and from that point onward, can only drink from that person alone. It is said that no blood taste sweeter for a vamp, than of their soulmates. Other blood simply turns to ash in their mouths. 

Having a short aged/mortal mate means dying of thirst when their partner dies. A vampires worst fear. Some vampires are naturally drawn to their mates and become obsessed with them, whether or not this is just the vampires nature or an effect from the witch curse is unknown. Nevertheless, their obsession can become detrimental to their sanity. But they can't afford to mate either as they know it will shorten their life span.

 Some vamps kill their mates before they succumb to forming the soulmate bond, but this can also destroy their mental health and make them obsessed with blood, as they know no other blood taste more divine than their mates. Those who do kill their mates, usually turn into mindless bloodsucking monsters, also called Leeches/Vile Bloods.