lala_sara

 So I'll be doing D&D module, or how it's called, based on Artto. It's 5e homebrew. It would be not detailed because I don't have means to test it, so just the outline. It's actually very helpful to make a D&D module on the world you write because then you have to think on mechanics that you probably missed in your worldbuilding.

So, first thing's first. The main difference would be the existence of divine insurance that gives everyone two pools of hit points (those are called charas on Artto). But well, D&D already have two pools - what do you think spell slots are? So my module explains the D&D mechanics more than it changes it. It still changes it though, especially healing process.
Next big change is the lack of other planes. There is a Reversed side that you can get onto. And there are other planets from where everyone except local flauna came but only archelves can go back and forth. Well, Atalanians also came back but that was permanent. So most of plane exchanges are not a thing for Artto.
Magic changes classifications. I don't like schools of magic, I don't like magic damage, nothing works right for Artto. Artto magic is like light - there is white one and there is a rainbow. Classification is about the process that the certain color does.
There would be changes in races but you still can take any race you like because Artto is convenient in the way that there can be anyone - there are firstcomers from all over the Galaxy, you can invent a race if you like.
Class differences are all over the place. There are some I don't change at all, some I change only in name, and some subclasses I would create just for Artto. Also most class names I twisted because why not.

So... measurements. Hit point is called chara. One spell slot for 1st level spell takes also one chara. 2nd level spell takes two charas and so on. Ki-point, sorcerer point and so on takes also one chara.

Everyone has two pools of charas: the main, life pool, and the second, insurance pool. They are different because life pool can only heal you, the second pool can be used for any other magic. First of all, of course, second pool was created to fill up your life pool if it gets empty, that's when insurance comes from. But then people started to realize that you can make magic from the second pool. But let's talk about first pool first.

There are no different rules for constructing a life pool. Most of classless people have 1-10 charas, classed people start to grow it according to rules. But it works differently. It's no longer an obscure thing. It's a pool of magic that your body can contain. When you are injured or tired or ill magic in it does the healing automatically. So the injury is healed but the pool missing charas now. For example - You have 21 hit points maximum, you've been damaged by 4 hit points - on Artto that means you were damaged on 4 charas to heal you. You have now 17 charas in your life pool but your injury is healed. So you can be at 1 hit point and still be totally fine because you are healed every time, automatically. So when someone heals you or you move charas from your insurance pool or take healing potion - you are filling your life pool for future healing, not to actually heal you right now. You already was healed, it's just if your life pool will be zero the next hit can kill you because you won't have charas to heal you. So you are going unconscious not on 0 but on minus equals the half of your maximum. For example if your max is 21 but you've been on 5, if you'll be hit by 5 you are still conscious and very much sound. If you've been hit 7 you are hurt but can roll constitution save to stay conscious. If you've been hit by 16 you're unconscious and start dying. 26 - you're instadead. Stabilizing rules are the same. Gods make resurrection possible so clearics (clerics) still can do all sort of things but remember that the body will dissipate if you're actually dead. So some spells for conservation of body are more helpful on Artto.
Life pool is filled during a rest by taking static magic from your surroundings. Rules are the same for short and long rest. It's a subconscious process and can't be done in places that have no static magic (magic-proofed chambers are the thing, used for all sort of things including preventing static magic gathering) but natural places like that are extremely rare. There are places that have magic tap with magic that you can take faster.
You can't heal or resurrect other people if you're not clearic or avatar (paladin). Gods have a monopoly on that. Medicine still works though! Body is still biological, you can try to stabilise and heal it with traditional care. But healing magic is either your automatic one from your life pool or it goes from gods. Healing potions are sold in temples. Self-healing as filling your life pool goes from the insurance pool. It will be done automatically when life pool on zero once per day, and every time you do healing, the only limit is the number of charas in your insurance pool.

Simple insurance pool is equal to your life pool. You are filling it by taking one chara every day from your life pool into it. You can't directly put static magic into it. Once the life pool goes to zero the insurance pool goes to life pool. Non-magic people also can do raw magic with insurance pool but it's risky. Only sourcerers (sorcerers) have their insurance pools as parts of their own bodies (and don't need gods to manage them) so you have to have some sort of an outside accumulator. It is called magem - magic gem. It is made from said parts of sourcerers. Non-magic people's magems are called prayers. They are small and usually uniform for certain gods. Magic people have their own unique magems that are bigger and better. They contain more than the life pool. And magic disciplines teach how to take more than one chara per day into it. Also wizards invented magic taps that you can put into that will get you more charas that you could gather from static on your own. Also there's batteries.

There is another way to count hitpoints and it doesn't need changes at all - see here

The magic user insurance pool is calculated like this: Number equal to the life pool + number equal to spell slots (1 chara for 1st level spell slot, 2 for 2nd and so on) of this class level and/or ki-, sourcery or other points + the same hit dice as with calculating life pool but plus your spellcasting modifier instead of constitution. First part is usual insurance that still will automatically go to life pool if it reaches zero, second is for spellcasting and the third part is additional. Additional part does not mean additional spell slots. Spell slots are from class' level which mean how many spells a magic user can do due to their training and skill, not the number of charas they have. Clearics and avatars have virtually unlimited insurance pools because they are linked to their god's pools but they still are limited in spells. But if your insurance pool have no charas spell slots won't help you - you still need charas to cast spells. So, for example, you still have 6th level slot but only 4 charas in the pool you cannot cast 6th level spell. Three parts are interchangeable, you can power spell slots with charas that would go to life pool and fill a life pool with your magic stock, it's just the maximum number of charas that your character can have. They rarely do though, because filling up both pools to maximum can be tricky.

Magic users have the ability to move any number of charas from life pool to insurance so filling up the insurance pool does not depend on this (there was a variant when you can move only number equal your level per day but that's unbelievably cruel I think. But as I can't test it...). Arcane recovery works differently than in original rules - with hit dice on short rest the same as with life pool but with spellcasting modifier and not constitution. So you have double of your hit dice - for life pool and for magic pool. The long rest will fill up your life pool but magic insurance pool you can fill up only with the tap (you can find it in wizard's houses or in places that host wizards often. The Kingdom have taps for free because they have wizard's union but it can cost money on other continents). The long rest for magic pool comes with the same set of hit dice. So you have one set of hit dice per day for short and long rest. The set returns after every long rest so if you need fast recovery you can use that day's set in the morning.