Thanks to the critique and help from others on Reddit, I've made some changes from v1.0 to add some more player agency to duels and a little more strategy. If I did my job right, this version should be a lot better!
Spell duels are a fun mechanic that shows arcane prowess in wizards and other casting classes. I've converted this mechanic to the Pathfinder rule set to 5E. For an example of what an awesome, high-level spell duel would look like, look no further than here (as long as you consider Harry a third combatant under the Frightened condition). A low-level spell duel that ends in a middle of a fight would be Harry vs. Malfoy.
This way of fighting is meant to be honorable, as a measure between wizards on how to settle conflicts -- with sheer magical power and cunning.
Starting a Duel
A spell duel is a form of combat, but unlike ordinary combat, the participants must all agree to willingly enter the duel and abide by its rules. If either side breaks the rules, it is considered the loser of the duel, regardless of any other outcome, and if its members continue aggressive action, the fight returns to the standard rules for combat.
The rules for a duel between spellcasters are usually very simple, but can be changed and altered by the participants, so long as both sides agree. Such discussions typically happen before the duel, allowing both sides to properly prepare, but as with all elements of a duel, this is not always the case. Most duels utilize the following simple rules.
A duel functions much like ordinary combat, with a few notable exceptions that make for a more exciting and challenging encounter.
Yield Check. At the start of the dueling round, combatants must confirm that they wish to start (or continue) dueling. If any one of the participants withdraws from the duel, the duel immediately ends for all participants. The participant or side that ended the duel is considered the loser of the duel. The duel's remaining participants can, among themselves, agree to resume the duel, but this is considered a separate duel from the previous one and does not involve those who withdrew from the duel.
Declare Spells. First, each duelist declares which spell they are casting at the same time. Though players may know what each other are casting, their characters do not. During "Resolve Spells" (detailed below), they can attempt to identify the opponent's spell.
Initiative Check. Next, each participant makes an initiative check, just like in standard combat. Because duels are always planned and expected, the surprised condition never applies. Duels in which one combatant unexpectedly strikes first means their opponent may declare the duel their victory at any time, but may not for any number of reasons (including wanting to show superiority over your opponent through arcane might).
Each duelist, before rolling initiative, chooses to Snap Cast or Simple Cast.
Snap Casting gives you advantage on your initiative roll but Arcana checks to identify your spell also have advantage. Wizards Snap Cast typically against either lesser skilled or less knowledgeable casters, using high-level spells when their opponents have little chance to recognize and counter the spell.
Simple Casting works as normal.
Resolve Actions. Each participant in a duel acts normally on their turn. Participants may even end their casting prematurely (called a feint, which doesn't consume the spell slot) and instead perform other actions (such as hiding!)
If they cast a spell, that spell must affect themselves, one of the other duel participants (whether this participant is an ally or an opponent) or the environment in a way that affects the duel in some way (for example, casting shatter to collapse the ceiling above an enemy).
In addition to the normal set of actions a dueling caster can perform each round, each participant in a duel may take a special counterspell action called a dueling counter, as noted below.
Dueling Counter. Each participant in a duel can take a special action once per round called a dueling counter. A dueling counter is similar to a counterspell, but is less specialized.
When a dueling opponent tries to cast a spell, the targeted spellcaster can make an Arcana check against the opponent’s Spell Saving DC as a reaction. If the check succeeds, they identify their opponent's spell and can attempt a dueling counter. If it fails, they cannot attempt a dueling counter against that spell.
To attempt a dueling counter, the countering duelist must expend a spell slot of a level equal to or higher than that of the spell being cast. The countering duelist must then make a check using their spellcasting modifier against the opponent's Spell Saving DC. The caster attempting the dueling counter receives a bonus or penalty on their check depending upon the level of the spell slot expended and the exact spell used, as noted in the table below. If the check is successful, the spell is countered — it is negated and the spell is lost. If not, the spell happens as normal.
Alternatively, a spellcaster can use Dispel Magic as a dueling counter. When a dueling spellcaster does so, they does not need to identify the spell being cast. When Dispel Magic is used as a dueling counter, it is not modified by any of the circumstances in the table below.
Counterspell works as normal.
Dueling Counter Modifiers
While duels can be treated as another form of combat, they are usually done to resolve a dispute between colleagues or rivals and are not usually intended to end in death. As a result, duels are usually fought with a specific prize in mind. Arcane academies are known for having duels to assign important faculty positions and as competitions between students for valuable prizes. In some places, magical duels of this kind are so common that special areas are constructed specifically for duels. Such dueling yards are sometimes made with special magic that can be activated for dueling competitions, preventing or reversing magic that instantly slays a foe or does permanent harm and automatically stabilizing creatures that fall to 0 hit points from damage.
In other places, especially arenas, institutions try to draw out these Spell Duels for as long as they can. To achieve this, their dueling yards are enchanted so dueling creatures either have resistance to magical damage or advantage on saves versus magical effects.
A man of few talents but many words, Pip began his foray into tabletop when he was a wee child and continues even now as a wee adult. He writes primarily for his preferred system: Dungeons and Dragons Fifth Edition!