Modifies Saving Throw to incorporate willpower and luck comparison on the saving throw effectiveness, so that a player with higher willpower and luck than their opponent will overcome their opponents' saving throw.  Saving throw messaging is improved to show if magic was fully saved against, partially saved against, or failed to saved against.  Now supports Vanilla Combat Event Handler when loaded afterwards.  Character advantage Bonus to Hit, and disadvantage Phobia, now have an effect on the saving throw.

In original gameplay, saving throw is a static 50%, with an increase of Willpower / 10, so most opponents will have a saving throw between 53-60, fully ignoring 30-40% of all magic.

This compares the player's willpower and luck against their opponent, modifying the saving throw based on the results.  Many opponents will have a lower saving throw based on their willpower, like bears, centaurs, ghosts, although some will have a higher saving throw based on their willpower until your willpower overcomes their's, like rats, orcs, daedra.
Saving throw reduces the effectiveness of spells within 50 of the saving throw, rather than 20, so that spells aren't fully resisted unless the saving throw roll is beyond 50.
If a character has an advantage for Bonus to Hit enemy type, that will now further increase your ability to bypass your opponents' saving throws by 25.  Likewise, having a phobia to an enemy type will further their saving throw by 20+(enemy level+4) i.e. a level 1 phobia has +25 to saving throw, while a level 20 phobia is a +44 to saving throw.
Innate Magic resist is determined by (willpower / 5 - 10 ), so it ranges from -10 to +10 depending on Willpower.

Formula is roughly:
50 + target willpower bonus + (target willpower - player willpower)/2 + (target luck - 50)/10) - (player luck - 50)/10
So, an example is that vs a skeleton warrior which has 40 WIL, a player starting with 67 WIL will have the following result:
Original saving throw: 54
New saving throw: 40

Due to how willpower is distributed on monsters, some opponents are quite resistant, like rats and bats, which appears to be the devs trying to hint not to waste magic on those.  Zombies are unusually resistant at 90 WIL also.

Disease and Poison will save using Endurance instead of Willpower, which is how it's described for the Endurance Attribute.  The target's endurance is compared to the caster's willpower when disease or poison is used.  This means poison and disease is more effective against low endurance targets even if they have high willpower.  This also applies to poisoned weapons and diseased creatures that endurance is used instead of willpower for resistance.

Overall, a player investing in willpower will be much more effective in magic vs opponents especially with destruction magic.

Available on Android

Credit to RedRoryOTheGlen for Vanilla Combat Event Handler support!

Formula Override Declaration for main mod
FormulaHelper.RegisterOverride(mod, "SavingThrowSpellEffect"
FormulaHelper.RegisterOverride(mod, "MagicResist"