Armour, Weapon, and Spell Mechanics

Armour Class

A character's normal armour class, or just armour class (AC), is their base AC as determined by their currently equipped armour (or their base unarmoured AC if they are not wearing armour, which is 10 unless stated otherwise for certain races or other situations such as using the mage armour spell) plus any Agility bonus that may be applicable to that armour, any bonus from a currently equipped shield, and any other bonuses that specify they are applied to AC.
A character's blind AC is determined by just their base AC as determined by their currently equipped armour (as specified above) plus any bonuses that specify they are added to blind AC, without any bonuses that require an action from the character, such as Agility bonuses or usage of a shield. A character's blind AC is used when they are , , , or cannot otherwise see or dodge and incoming attack.
A character's touch AC is determined as their unarmoured base AC plus any bonus from a currently equipped shield, and any other bonuses that specify they are applied to touch AC. A character's touch AC is used when they are targeted by something that does not need to get through their armour to affect them, such as many touch spells.

Weapon Modifiers

An attack made with a melee weapon receives a to-hit bonus equal to the character's Strength modifier, plus their proficiency bonus if the character using the weapon is proficient with it, plus or minus any other modifiers (such as attacking a Flanked creature or one under the effects of the blur spell). A successful melee attack receives a damage bonus to the roll equal to the character's Strength modifier. Weapons with the Finesse property may use Agility for both the to-hit bonus and damage bonus.
An attack made with a ranged weapon receives a to-hit bonus equal to the character's Perception modifier, plus their proficiency bonus if the character using the weapon is proficient with it, plus or minus any other modifiers (such as attacking at an extended range). A successful ranged attack receives a damage bonus to the roll equal to the character's Perception modifier. Ranged weapons have three ranges, the first is short range, and incurs no to-hit penalty if used within this range, the second is medium range and incurrs a −3 to hit if used within this range, and the third is long range, and incurrs a −6 to hit if used within this range.

Spell Modifiers

Some character classes get access to spellcasting. Some things, such as spell lists, mana gain, and total number of spells known, differ between classes, while some mechanics are universal; this article serves to explain the universal mechanics of magic.
Each class has their own spell list. When leveling, a character gets to learn a new spell or spells from their class' spell list. However, some spells are considered taboo or forbidden, and thus cannot simply be learned via leveling. Spells like this (marked by an asterisk in the spell list) may only be learned after a character has acquired a spell book containing the forbidden spell.
When casting spells that affect other creatures, the spell will most likely require an attack roll, or a saving throw. A character's to-hit bonus with a spell is equal to their proficiency modifier + their Willpower modifier. A character's spell save DC is equal to 9 + their proficiency modifier + their Intelligence modifier.
Cantrips do not require any mana to cast. At a base level, first level spells require 5 mana, second level spells require 10 mana, third level spells require 20 mana, and fourth level spells require 40 mana. Some things can affect these amounts, such as perks that reduce the mana costs of certain spells, or using metamagic. Most spells can be up-casted to a higher level, incurring the normal mana cost for a spell of that level.