What is Arcanum
Beyond the fundamental forces which govern reality lies a force wholly un-natural. The ability to create matter rather than simply change it. The power to manipulate mind, body and spirit — the unnatural art of the arcane.
Arcanum is not merely a tool. It is a transgression against the natural order — one with power enough to reshape the world, and consequences enough to unmake it.
Below are the core concepts that govern how the arcane works, what it costs, and what it demands of those bold enough to wield it.
Arcane Contamination
Spells and other applications of arcane knowledge cause the very nature of reality to fray at its seams. The corruption that follows is inevitable — only its severity varies.
The magnitude of the spell. Greater potency tears more deeply at the fabric of reality.
How often the arcane is invoked. Repeated use compounds corruption in an area over time.
How long the caster and their surroundings are exposed. Prolonged exposure worsens contamination steadily.
Areas can become corrupted by the arcane to varying degrees, with effects and consequences that range from benign to devastating.
Pure Magic
Some magic is mastered in such a way that its use produces no corruption and costs no mana. These spells are referred to as pure magic.
Only the most powerful of mages can turn powerful spells to their pure form. Pure magic represents the apex of arcane mastery — an understanding so complete that the caster imposes their will upon reality without resistance, leaving no wound in its wake.
Potency
All spells can be rank ordered on the magnitude of their effect and the proportional cost in mana they command. This measurement of magnitude is called potency.
A spell caster can only cast spells whose potency level is equal to or lower than their mastery with arcanum.
Potency serves as both a measure of a spell's raw power and a gating mechanism — spells of potency beyond a caster's mastery remain simply beyond their reach, not merely difficult to cast, but inaccessible entirely.
Mana
To perform their otherworldly effects, mages tap into an innate, cultivated resource: mana. Mana is a replenishing resource pool that spell casters draw from when casting spells.
Mana Pool
Your mana pool represent the total amount of mana that you have per day, resetting upon sleep. Your mana pool increases with you as you level up, being equal to your Character Level + Arcanum Mastery. Class features, talents and other effects may also increase your mana pool.
Mana Pool = Character Level + Arcanum Mastery Level
Mana Cost
Spells that are not pure cost an amount of mana to cast equal to their potency.
Mana Cost = Potency of the spell
Spells Known
Spell casters gather the ability to cast new spells — whether through a pact with a powerful being, through dedicated study, or through their own innate capacities. The spells known by a caster are the only spells they can cast. If a spell is not found within that list, it cannot be cast.
The Potency Rule
No matter how many spells you know, a player cannot know a spell they cannot cast — meaning they cannot know a spell whose potency is greater than their arcanum mastery level.
How many spells a character learns is influenced by many features, however the class which provides a player with arcanum mastery is the primary determinant for spells known.
Spellcasting Attribute
The usage and mastery of the arcane leverages a character's innate capacities. The spellcasting attribute of a player will be either Will or Intellect, depending on their starting class selection or arcanum-providing feature.
Driven by resolve, faith, and force of personality. Magic shaped by the soul.
Driven by study, logic, and arcane theory. Magic shaped by the mind.
This attribute is used for many elements of the arcane: determining thresholds, adding to spell attacks, scaling damage, and increasing mana depending on class features.
Spell Attacks
Some spells are attacks. When a spell is an attack, the attack bonus (to hit) for that attack is equal to the spellcasting attribute of the casting character.
Spell Attack Bonus = Spellcasting Attribute
What counts as an attack?
Spell attacks are considered attacks for the purpose of all features and effects that reference 'attacks'. They would not be affected by features which reference specific attack types — for example: 'weapon attacks' or 'attacks with a shortbow'.
Ritual Magic
Some magic is controlled, slow, and requires setup, tools, and time. These spells are long-winded affairs involving many different elements, and are called rituals.
Cannot be performed in combat
How long the ritual takes to complete
How long the effects last once the ritual is complete
None — their slow and controlled nature prevents corruption
Any effects listed in the ritual will expire once the spell duration has elapsed.
Channelling
Some spells take time to complete — not as long as a ritual, but the process extends over multiple turns as you channel the magical effect through you. Channelling spells have a duration listed in turns, referring to how many turns you need to spend channelling the spell.
A turn is considered to have been spent channelling when you first cast the spell, and whenever you subsequently choose to channel.
How it works
1. Cast the spell. You gain the channelling condition. This prevents you from performing any action other than basic movement actions, and prohibits any out-of-turn AP use.
2. Each subsequent turn. At the start of each turn after resetting your action point total, you are given the option to channel. Choosing to do so requires spending the AP cost of the spell again (you do not spend the mana again). Doing so increases the turns spent channelling by 1, and the channelling condition persists.
3. Finish channelling. You finish channelling once the turns spent channelling equals the duration of the spell.
Spellcasting Threshold
Many spells cause harm or inflict deleterious effects upon their targets. To resist these, creatures will often be prompted to make a resistance check of a particular type. The threshold used to determine success or failure of that check is called your spellcasting threshold.
It is influenced by many features, but primarily your spellcasting attribute. Your base threshold is:
Spellcasting Threshold = 8 + Spellcasting Attribute
This value scales naturally as your spellcasting attribute grows, meaning that more powerful casters become progressively harder to resist.
Reference
A quick-reference summary of all key values and formulas used in arcanum. Keep these in mind during play to resolve arcane interactions quickly.
Equal to the potency of the spell
Pure magic costs no mana
8 + Spellcasting Attribute
Base value; may be increased by features
Spellcasting Attribute
Determined by class and features
Cannot know spells with potency above arcanum mastery
Character Level + Arcanum Mastery Level
Replenishes on sleep
Equal to arcanum mastery level
Cannot cast spells of higher potency
Caused by potency, frequency, and duration of arcane use
Rituals and pure magic produce none
Spell AP cost paid each turn channelled
Mana is only paid once on initial cast
Will or Intellect
Determined by starting class or arcanum-granting feature