Magic

After the fall, in an attempt at reconciliation between the gods and the Source Races, Vimdos changed the weave she had created after the settlement of Amarynth.
From now on, powerful magic would require sacrifice from the casters. This meant to keep Magic in check and prevent another cataclysm like the Fall.

Mechanically speaking all leveled spells have an HP cost. This damage is unreducible and ignores all resistances and immunities. Regardless of what is attempted, the caster of the spell will incur this damage.
This damage will never bring a caster below 1hp but upon casting ANY (leveled or cantrip or spell scroll use) spell at 1hp causes additional effects.

Spells cast follow the optional rule in the 2014 DMG: Spell points

"With this variant system, a character who has the Spellcasting feature uses spell points instead spell slots to fuel spells. Spell points give a caster more flexibility, at the cost of greater complexity.

In this variant, each spell has a point cost based on its level. The Spell Point Cost table summarizes the cost in spell points of slots from 1st to 9th level. Cantrips don't require slots and therefore don't require spell points.

Instead of gaining a number of spell slots to cast your spells from the Spellcasting feature, you gain a pool of spell points instead. You expend a number of spell points to create a spell slot of a given level, and then use that slot to cast a spell. You can't reduce your spell points total to less than 0, and you regain all spent spell points when you finish a long rest.

Spells of 6th level and higher are particularly taxing to cast. You can use spell points to create one slot of each level of 6th or higher. You can't create another slot of the same level until you finish a long rest. (Rule changed, See addendum below)

The number of spell points you have to spend is based on your level as a spellcaster, as shown in the Spell Points by Level table. Your level also determines the maximum-level spell slot you can create. Even though you might have enough points to create a slot above this maximum, you can't do so.

The Spell Points by Level table applies to bards, clerics, druids, sorcerers, and wizards. For a paladin, ranger (or Artificer), halve the character's level in that class and then consult the table (round up). For a fighter (Eldritch Knight) or rogue (Arcane Trickster), divide the character's level in that class by three (round down)."

Warlocks will be discussed below.
Any feature that recovers spell slots will recover a corresponding amount of spell points instead.

Important change to this ruling: You are not limited to one spell of 6th level or higher per day.

The damage incurred follows this table:

Level of the Spell	Damage incurred upon starting spell cast (ignores temporary hit points)
1	1d4+3
2	1d6+6
3	2d6+9
4	2d8+12
5	3d8+15
6	3d10+18
7	4d10+21
8	4d12+24
9	Brings the caster to 1 hit point.

Casting a spell (cantrip, leveled or spell scroll) at 1 hit point

The caster will, upon starting the spell casting process need to make a CON saving throw.
The DC for this roll equals 11 + spell level.

Upon success, spell is cast successfully, the spell point cost is halved but the caster incurs 1 level of exhaustion.
This exhaustion can only be removed by a long rest.
To avoid confusion we will use the term "Spell Fatigue". It follows the 2024 exhaustion table.

Upon failure, the caster falls to 0 hit points and must roll 1 death save immediately.

DM note:
I highly recommend traditional casters to invest in constitution if they plan on casting high leveled spells often. Proficiency in CON saves and ways to buff saves will likely be a big boon.

WARLOCKS
Warlocks do not take take their powers from the weave and gods so are not subject to the hit point cost of casting spells.
Considering the math basically works out the same with spell points or traditional spell slots and they are not capable of casting spells over 5th level 'normally' anyways; I chose to let warlocks use their regular spell system.
Example given:
Level 17 spellcaster gets 107 spell points. Warlock lv 17 has 4 lv5 spell slots and mystic arcanum 6-7-8-9. Say you get 1 short rest during the day, this works out to (4x7)*2+9+10+11+13 = 99. This seems fair enough considering just taking 2 short rests gives you over the amount of spell slots possible in spell point system.
Reminder, they do not take the hit point cost occurred with leveled spell casting.