Dramatic Mage Duels in D&D 5e

I shadow-DM my little brother's D&D game, helping him plan story arcs and dramatic battles. His next session will be a boss fight, where the sorcerer PC will be facing off against their evil spellcaster mother. He thought it would be both dramatic and thematic for the two of them to fight in a "mage duel" while the rest of the party fought the enemy horde, which sounded bad-ass to me. But how should we represent this?

Right off the bat, "just use normal spellcasting" was out. It's boring, and would largely just be a matter of "try to cast, get counterspelled, they try to cast, you counterspell, etc" until one of them runs out of high-level spells. Boring and frustrating, far from dramatic, and in the meantime the other characters might be actually beating up the mom in melee, which isn't thematically appropriate either.

Bro recalled that Mage Hand Press, his favorite 5e imprint, had published a simple Mage Duel ruleset, as part of a larger set of "wizarding contests". This looked like a good starting point, but it was clearly designed to be a fast, simple resolution mechanism as a small part of a larger roleplaying session around a wizarding carnival or tourney. It really didn't have any strategic depth (just choose your highest-level slot each time and hope you roll well), which didn't suit our dramatic boss fight.

So we iterated on this a bit, and ended up with something that I think will be quite fun at the table. It has some actual strategic choice, rewards predicting the opponent's moves, and lets the duel have an effect on the rest of the battle being waged around them.

Dramatic Mage Duels!

At the start of each round, before anyone else takes actions, the two duelists secretly choose a spell slot they want to expend, and a dueling stance. They reveal both, then make an opposed Arcana check, using their spellcasting ability. Each duelist gets a bonus to this check equal to half the chosen slot's level (round down, as usual). The loser of the opposed check takes (winner's slot)d6 damage. A tie causes both characters to take (their slot)d6 damage, as the spells rebound.

There are three dueling stances: attack, shield, and tricky. The two opponent's stances are compared in a rock-paper-scissors fashion: attack beats tricky, tricky beats shield, and shield beats attack. If one of the characters win, they get advantage on the Arcana check. (Ties have no effect.)

In addition to the direct damage the duelists deal to each other, the chaotic energies of their duel spill out and buff/debuff the rest of the combatants. Each duelist also makes a roll on the Chaotic Buff and Chaotic Debuff tables, adding their expended slot's level to the Buff roll and subtracting it from the Debuff roll. Everyone else on the duelist's side of the combat benefits from the rolled buff and suffers the rolled debuff.

Finally, the chosen stance has additional effects. If you choose Attack, you deal 3d6 damage to your opponent (whether you win or lose the Arcana contest). If you choose Tricky, you roll on the Buff table twice and choose the result you prefer. If you choose Shield, you roll on the Debuff table twice and choose the result you prefer.

The duel is over when either duelist yields, or dies.

Buff And Debuff Tables

These are taken with only slight modifications from the very nice Sorcerer tweak by SwordMeow (direct link to PDF), which presents a much more interesting Wild Magic Sorcerer than the original.

Party Buffs
d20	Effect
1	No effect.
2	A pleasant smell fills the air around the party.
3	Small plants grow to bloom around the party's feet.
4	The party's clothes become cleaned, and faded dyes return to full brightness.
5	The party becomes sheathed in a thin layer of protective energy. They each gain temp HP equal to the spell's level + your spellcasting modifier.
6	The party is innervated. Their movement speed increases by 15' until the start of your next turn.
7	The energy of the spell flashes brightly around your party. On each party member's turn, an enemy of their choice within 5' must make a Constitution saving throw against your Spell Save DC or be blinded until your next turn.
8	The energy of the spell swirls around your party and sheathes their weapons. Until your next turn, they can choose to have any damage they deal become Force damage instead.
9	With a surge of energy, your party is able to shift their position. During each of their turns, your party can teleport up to 10' as a free action.
10	Arcs of chaotic energy emanate from your party. A random hostile creature within 15' of each party member takes 1d8 lightning damage.
11	Enemy attacks are pushed away. The next attack against each party member before the start of your next turn is made with disadvantage, or the next saving throw each party member makes before the start of your next turn is made with advantage, whichever comes first for each party member.
12	The air around your party crackles. Until the start of your next turn, if a creature with 30' of a party member hits them with an attack, they can use their reaction to deal {level of the slot}d4 lightning damage to the attacker.
13	At the start of each party member's next turn, they roll on this table and gain the effect on themselves only.
14	Your party becomes energetic. They gain the effects of the Haste spell until the start of your next turn, ignoring the lethargy.
15	Your party's strength amplifies. Once during each of their next turns, they can add {level of the slot}d6 to the damage of an attack or ability they make, of one of the damage types already dealt by the attack.
16	If a party member takes damage from a spell before the start of your next turn, they can roll 1d8 and subtract the result from the damage they would take.
17	Your party's defense seem to deflect the enemy's attacks. Until the start of your next turn, all attacks made against your party have disadvantage, and all saving throws your party members make have advantage.
18	Your party's attacks seem to guide themselves to their destinations. Until the start of your next turn, your party gains advantage on all attack rolls, and impose disadvantage on all saving throws their opponents make.
19	The spell is efficient, allowing you to siphon excess magic. You regain the spell slot you expended for the duel this round.
20	Roll twice on this table and gain both effects, rerolling additional 20s.

Party Debuffs
d20	Effect
1	Roll twice on this table and suffer both effects, rerolling additional 1s.
2	The spell requires additional energy to cast. You must expend another spell slot of the chosen level, or else you automatically lose the opposed Arcana check this round. If you have the Font of Magic feature, you can choose to instead expend sorcery points equal to the slot's level.
3	The party's movements become erratic and difficult to control. Until the start of your next turn, they have disadvantage on any attack rolls they make, and enemies have advantage on saving throws against their effects.
4	The party's movements seem to have them dodging directly into enemy attacks. Until the start of your next turn, enemies have advantage on all attacks against your party, and your party has disadvantage on all saving throws.
5	If a party member takes damage from a spell before the start of your next turn, they take an additional 1d8 damage of a type the spell already deals.
6	The party's energy bleeds away. If they deal damage before your next turn, the damage decreases by {level of the slot}d6.
7	Electricity courses thru your party's muscles, causing them to tense up. They suffer the effects of the Slow spell until the start of your next turn.
8	At the start of each party member's next turn, they roll on this table and suffer the effect on themselves only.
9	Static energy builds up in your party, waiting for something to ignite it. Until the start of your next turn, if a party member is damage by an attack, they take an additional {level of the slot}d4 lightning damage.
10	Enemy attacks are drawn to your party. The next attack on each party member before the start of your next turn has advantage, or the next saving throw they make against a hostile effect has disadvantage, whichever comes first.
11	Arcs of chaotic energy emanate from your party. A random friendly creature within 15' of each party member takes 1d8 lightning damage.
12	Your party's position becomes unstable, and they blink side to side wildly. Each party member immediately teleports to a random unoccupied location 10' away from their current location.
13	Your party's attacks are blunted, unable to penetrate defenses. Until the start of your next turn, all enemies with resistance to some type of damage instead have immunity.
14	A flash of light bursts in each party member's face. They must make a Constitution save against your Spell Save DC or be blinded until the start of your next turn.
15	Your party is enervated. Their movement speed decreases by 15' until the start of your next turn.
16	Some of your spell's energy backfires. Each party member takes force damage equal to the slot's level + your spellcasting ability modifier.
17	The party suffers a faint ringing in their ears for the next minute.
18	Fractal, lightning-like scars appear on your party's hands and arms, which disappear when they receive any magical healing.
19	Your party immediately becomes thirsty and hungry.
20	No effect.