EDIT: I guess this got moved, because of the intro. That's fine. But this is about a component of the setting, the story is just to demonstrate it in action.

Imagine a scene. You're walking down a city street late at night, in that hazy period between most people going to sleep and the criminal class setting themselves to work. The only people that should be out now are women of the night, which raises the question of what you're doing out, but that's your business. But as you progress towards your unknown destination, you hear a sharp, but muffled sound from an alley before you. You can't help but take a look.

There, you find a priest standing in the alleyway. His hat has fallen to the floor, but his holy vestments and the symbol of the eternal flame are unmistakable. His nose is broken, and one of his eyes is swollen, but these injuries are healing before your eyes. In his arms, one of the aforementioned women of the night, his lips to her neck, trying fruitlessly to push him off. Blood runs down his chin, staining his white collar. She goes limp from blood loss, and the life drains from her eyes as you stare at the scene.

Soon, he draws back from her neck, licking blood from his lips. He stares for a moment at her lifeless body, laying her down on the ground. He places her arms on her chest, and puts a paper flame in her hands, much like his own holy symbol. He picks up his hat, and turns to leave. He only spots you then, shoving you out of the way as he flees down the street, but for some reason your eyes can't track him as he moves. As he vanishes down another street, your eyes are drawn back to the woman's lifeless form, but only one question dominates your thought.

What have you just witnessed?

A vampire? Perhaps he is that far gone. But then, it seems likely if he was he would have attacked you as well. Perhaps he is a vampire, and a coward who only attacked a woman not able to adequately defend herself. Clearly, it's not because she injured him, the very idea seems laughable seeing what little damage was visible healing so quickly. Maybe he simply had his fill. But no, if there's a "fill" for him to have, he probably isn't a full vampire yet.

More likely, this was a vitae user replenishing his stock. But if that was the case, normally he wouldn't have killed her. Especially given he was a member of the clergy, he should have had coin to pay for blood. She's probably done that before, given her profession. Perhaps he killed her to protect his holy reputation? Or perhaps he got carried away once he tasted her blood. If so, he is well on the way to becoming a proper vampire. She does have a bite on her wrist as well, that's got to be it. In any event, it is time to leave, before a bobby comes along and thinks you've done this. They'd never believe you if you told them the murderer was somebody so far above suspicion as a man of the cloth.


Now that the intro is out of the way, it's time to actually explain what vitae is, how it works, and what it has to do with that man being a blood-sucking parasite. Also, why he's becoming a vampire.

Vitae is a magic resource in Aelsif, while nobody's actually sure what it is per se it's believed to be a sort of life energy. Hence the name, "vitae". It's partially replenishing, generating itself up to a tiny number, based on the user's constitution, regenerating only as long as the user is at full HP and taking an entire day to get back each point. You can have far more than this, but you'd need to acquire it somehow. And by the intro, you know how that's done. By default, vitae isn't important enough for that. It's just an emergency backup for your normal magic resources that is dipped into when they run out, because when it runs out you start spending HP instead. This is enough for casual users, but some people invest perks into expanding what vitae is able to do, so they need a lot more of it. And for them, how to replenish it and acquire more than their natural amount is important. Vitae is stored in the blood, though most of it isn't usable to its owner, and that means the easiest way is via haematophagy as seen above. This is the basis of a number of things, ranging from necromancy to resurrection, dark damage and blood magic, all of which require vitae to function.

Clerics are FAR more vulnerable to this, as many of their miracles directly require vitae since they can produce all of the effects that need it. They're also the most voracious users of normal casting resources and their religions often have rules about some of the consumables normally used to restore them, causing them to dip into emergency vitae use more often than other archetypes. So, vitae investment seems like a natural choice to augment their class abilities, and once you've started it's easy to keep going.

However, once you have started investing perks into it, vitae can quickly become your most important resource, more important than your silver, and we all know people will kill for that. It isn't too long after that it becomes a physical addiction. These perks often come with a downside, and these may start sounding familiar. Gaining more vitae, but no longer regenerating it. Gaining even more vitae, but it gradually depleting at a point each day. Being able to injure yourself and drink your own blood to restore it. Gaining a hefty resistance to dark damage scaled to your vitae, but effects that target unholy things will affect you. Gaining temporary damage reduction that is disabled by silver weapons. Weakness to fire in exchange for a resistance to all other elements. Occult magic usable only at night, but the sunlight has negative effects on you. There's a wide variety of abilities you can gain, one spent perk at a time, some with downsides and some just requiring a lot of vitae.

If these are sounding like the powers of a vampire, that's because that's exactly what they are. Vampires aren't a separate thing in Aelsif, they aren't a kind of creature, they're the term used for those who have invested in vitae so heavily they become enthralled by their need for blood and it takes over their life completely. There's a vast gulf between most vitae users and the kind of crazed, bloodthirsty cannibals the term applies to, but even so it's easy to understand why vitae is seen as an unholy power. And why it being seen as unholy is ironic, given how priests are by far the most drawn to its succour. This may even be the reason why Marakiziy priests involve red wine in their rituals, since it allows them to easily disguise their real intoxicant. Or perhaps, they simply use wine because alcohol is useful to casters in and of itself. It's such an old practice, nobody can say for sure why it's done.

Despite the danger, vitae isn't necessarily a bad thing to use if you're smart about it. It's a good source of power and builds relying on it are capable of being quite powerful. It's ultimately up to you to decide if the reward is worth the risk. So pour yourself a glass, and think it over. Let me know what you decide.