Magic is going to be the big thing that acts a bit differently in this campaign. It will diverge quite a bit from the magic stuff in the PHB. So we're going to outline a few things about magic and how it works in Fendojon and some of the changes to magic in general and the classes that will use it.
The magic of Fendojon stems from the Numen, their very presence fills the continent with their energies. Over time humans have learned to sense and control this energy with their willpower. Spellcasters learn to sense and detect this energy and latch onto it with their wills and shape it into a specific desired effect. Unlike other areas of Hekinoe, the magic of Fendojon does not cause misfires, physical mutations, or lead to exploding magic items.
Magic can do many things, but there are several things it cannot do. Period. Regardless of caster level and ability scores. There are no planes, so it cannot be used to travel to other planes or contact creatures from other planes. Since there are no other planes, it cannot summon creatures or objects from other planes. It cannot bring back the dead or create undead. There are no gods on Hekinoe or in my campaign universe (though there are extremely powerful supernatural entities), so it cannot do anything involving holy power or contacting a deity. It also cannot create life. This last part is relevant because in the Pathfinder and GURPS versions of Hekinoe summoning type spells did not actually summon planar creatures, they used sorcerous energy to create living creatures made of magic. The magic of Fendojon cannot do this.
The magic of Fendojon is also impermanent. It doesn't wear out or run out, it just returns to the grand mass of magical energy that suffuses Fendojon. The maximum limit on how long a spell will last in Fendojon is equal to the caster's Wisdom modifier. So mage armor can still last up to eight hours, but imprisonment, even though its normal duration is until dispelled, cannot last longer than a number of days equal to its caster's Wisdom modifier. This obviously limits the usefulness of some spells. This also means that spells that can be made permanent through repeated castings, like guards and wards, cannot be made permanent.
Because using magic is not a learned skill and relies entirely on a caster's ability to control and shape magic with their willpower, all spellcasting is done via Wisdom, not Intelligence or Charisma. So any time a Wizard would normally use his Intelligence modifier, he instead uses his Wisdom modifier. The same goes for Bards, Eldritch Knights, Arcane Tricksters, feats that grant spellcasting ability, and so on.
One of the very very major changes to magic is that all spells, except those that do something magic cannot do (animate dead, commune, raise dead, plane shift, etc, etc, etc) are available to all casters. So Wizards can use cure wounds, Bards can use fireball, and Druids can use eldritch blast if they want to. Bards, Druids, and Wizards will all still have a variation of spells known and spellbooks, but this mechanic will represent a focus of theirs, rather than the sum of their abilities. They may actually all end up with a spellbook mechanic like the Wizards for the sake of simplicity on my end. Dunno yet. We'll see.
This spellbook/preparation/known spells mechanic is going to represent a kind of mental muscle memory. You train your body to automatically do something a certain way. This known/preparation mechanic represents magical castings that are so second nature to the caster that they require little to no effort on their part to cast properly. So what happens when a caster goes outside of their known/prepared spells? The first thing is that they lose their proficiency bonus on the DC and attack rolls of the spell (assuming the spell has such mechanics). The second thing is that the spell costs them more spell points. One step higher in cost specifically, so a first level spell a caster is unfamiliar with would cost as much as a second level spell. A first level unfamiliar spell augmented to third level would cost as many spell points as a fourth level spell, and unfamiliar cantrip would cost as much as a 1st level, etc, etc. This is somewhat limiting. Since your level limits the level of spells you cast, the increased level/cost of unknown/unprepared spells will delay your ability to cast certain spells if they are not spells you're familiar with. Don't worry. This will be explained more intuitively in documentation for this campaign.
The mechanic I'm going to utilize instead of spellbooks and spells known/prepared is going to be Wisdom based. You'll start off "knowing" spells equal to your Wisdom modifier plus your proficiency bonus. So casters won't "know" a lot of spells, but they'll be able to use all of them on the fly, albeit at reduced efficiency. In addition to this, all spellcasters will be able to cast detect and dispel magic at will as if they were one of their "known" spells. I may institute certain specialties allowing spellcasters to add more spells to their "known" list. Like Wizard (Transmuter) adding transmutation spells or a Fighter (Eldritch Knight) adding abjuration spells. We'll see.
Fyi, I'm considering instituting something like mental fatigue for when you spend most or all of your spell points. It's actually extremely likely. Spell points do not represent a spellcaster expending his own internal magical energies to cast spells. It represents his skill in drawing and directing larger amounts of magical energy from the magic suffusing Fendojon. What he's really expending to cast spells is his mental energy and strength of will. When he runs out of spell points, it's too hard for him to focus his will, similar to how workout nuts are exhausted after they go into beast mode on leg day.
The next relatively major change is that spells do not require verbal, somatic, or material components in Fendojon. This may seem somewhat overpowered, but it's a universal change not limited to just characters. If a spell requires you to interact with the world or your target in some way, like touching them or spitting at them or screaming at them, that is unchanged, so having your hands bound or being mute may still be relevant.
The magic of Fendojon is flavored by the thoughts, emotions, and personality of their wielder. Their spells are flavored with this as well and they develop and aura that can be sensed by other casters. So the spells of a very stoic and reserved caster of earth magic will feel like silence shrouded in wall of stone. A very disciplined, but passionate, caster of earth magic might have an aura that feels like the heat of lava at the heart of a long dormant volcano. Because all spellcasters are able to sense and direct this magical energy, they can sense these auras innately and have advantage on all Wisdom (Insight) and Wisdom (Perception) ability checks against spellcasters, the same goes for magical creatures and plants, but that would fall under Wisdom (Animal Handling) and Intelligence (Nature) instead of Wisdom (Insight). This does have the added effect of adding a signature of sorts to a spellcaster's spells, which makes it potentially easier to identify who did what to whom with magic.
Magic items will also operate a little bit differently as well due to the nature of magic on Fendojon. Due to the sheer amount of magic on Fendojon, they will be cheaper and more common. However, permanent items will be something of a pain in the butt to use. As I said, the magic of Fendojon is not permanent, so eventually the magic of enchanted items leaks back out into the world. Magical items with normally permanent effects purchased in stores will automatically have five days of charge left, those looted from fallen foes or stolen from random passersby will have 1d4+1 days of charge left, while those looted from long derelict tombs and such will be empty. An empty magical item will have an aura left on it as easily detected as that of a normal magical item, so discovering magic items won't be terribly difficult. This aura, an echo of the item's enchantment, just needs to be topped off with a Wisdom + proficiency bonus check with a DC dependent upon the item's rarity: 12 for common magical items, 14 for uncommon, 16 for rare, 18 for very rare, and 20 for legendary. The spellcaster also needs to spend spell points equal to a spell of a certain level to recharge the item: 1st level for common magical items, 3rd level for uncommon, 5th level for rare, 7th for very rare, and 9th for legendary.
Alright, so I think I've covered the majority of the issues regarding magic in Fendojon. It's still all broad strokes and rough ideas at this time, so it's not really juicy mechanics and stuff, but that will appear at a later date. Probably.