Group Spellcasting: D&D 5E Homebrew Challenge

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Homebrew Challenge is an occasional column in which we develop a homebrew solution for Dungeons & Dragons 5th Edition in order to create additional player options, actions, or abilities. We discuss the challenge, the solution, the design principles we employed and ideas that we rejected, and the areas of concern that Dungeon Masters should keep an eye on.

The Challenge: Group Casting

The idea that a group of casters can combine their power into a single spell is as old as magic. It shows up in fiction in various forms, from Shakespeare’s witches in Macbeth to a variety of psykers, heretics, and cultists in the Warhammer 40k universe. The goal here is straightforward: create a mechanic by which two spellcasters can combine their spells to achieve a greater net effect. 

The Solution

The solution is to create two new actions in combat in addition to the options presented on pages 192-193 of the Player’s Handbook, with the intent to allow one caster to begin casting the spell on his turn, and a second caster to complete the casting on her turn, with a greater effect than if they had cast the same spell separately.

Ready a Group Spell

When you ready a group spell, you begin casting it but hold its energy to channel into an ally’s casting of the same spell. The Ready of Group Spell action is a Ready action (see page 193) with the trigger of “An ally casts an identical spell”. In addition to the normal restrictions for readying a spell, the spell must have a casting time of 1 action, a duration of Instantaneous, must cause damage to its target, and must not be a cantrip.

When the trigger occurs, the spell effects are resolved as part of the Complete a Group Spell action.

Complete a Group Spell

When you complete a group spell, you finish casting a spell that your ally began. You may only use Complete a Group Spell if an ally you can see within 30 feet has taken the Ready a Group Spell action. When you take this action, you cast an identical spell as your ally cast with the Ready a Group Spell action. If the spell can be cast at different levels, you must use the same level. If the spell was cast using Metamagic, you must use the same Metamagic. Treat this as if you had cast the spell normally, with the following exceptions:

  • If the spell calls for a spell attack roll, rather than the normal damage for the spell, multiply the number of damage dice by 2.5, rounding up. For example, if a spell normally does 5d6 radiant damage, it instead does 13d6 radiant damage.
  • If the spell calls for a saving throw, the target has disadvantage on the saving throw. Rather than the normal damage for the spell, multiply the number of damage dice by 2. For example, if a spell normally does 8d6 fire damage, it instead does 16d6 fire damage.

Design Principles

The idea was to keep the mechanics simple: the first caster maintains concentration on a spell, the second caster resolves the spell, and the effect is slightly greater than if they had cast it separately. It’s a basic risk-reward proposition, and the intent is to prevent the effect from being so great—or the risk so small—that the casters always attempt group casting.

For the first caster, the risk is that his spell (and his spell slot with it) will fizzle without ever resolving, either because he loses concentration or because his target doesn’t complete the casting. This is a variable risk dependent on the initiative order in combat. Players will be less inclined to use group casting if enemies act between their turns, and more inclined to use group casting if they act close to each other in initiative order. For this reason, the benefits should be relatively limited.

There are some hidden costs to this action as well. Because the Ready a Group Spell action counts as casting a spell, cannot be used on a cantrip, and requires the caster’s reaction, this prevents the first caster from casting any other spell during the round (note that the wording of the Bonus Action description on page 203 prevents a Sorcerer from casting a cantrip as a bonus action using Quickened Spell metamagic.) Additionally, two spellcasters must know and prepare the same spell at the same level, which reduces the party’s overall versatility. Finally, unless both are Sorcerers, they will not be able to use Metamagic to boost the spell’s effects.  In return for those costs, the casters combine their spells into a single effect, granting a bonus to the damage of some spells, and imposing disadvantage on the saving throws for others.

In the interest of conserving mechanics, I also considered using some form of the Help action. Since that doesn’t consume resources and spell slots, it could only provide marginal benefit. That benefit would hardly be preferable to simply casting the same spell twice, separately.

Areas of Concern

It’s important to keep an eye on the damage multiplier effect at your table, since more damage from a single source has wonky knock-on effects in D&D 5th Edition. Concentration is the most obvious such mechanic. Take two Fireballs, both with failed saves, for 8d6 damage yielding an average of 28 fire damage; this results in two concentration checks, each with a very makeable save DC of 14. Now take one group Fireball with a failed save for 16d6 damage, yielding an average of 56 fire damage; this results in one practically impossible concentration check with a save DC of 28.

Additionally, any effect which reduces damage by a fixed amount (such as the Heavy Armor Master feat) is less effective because it triggers only once. More concerning, if turned against the PCs, a single group spell is liable to trigger Instant Death, even at middle levels; since enemies tend to go on grouped initiative, it’s unlikely the players will have a chance to interrupt such a group spell.

If the massive damage proves disruptive at the table, there are some ways to raise the costs. Consider requiring both casters have a feat before they can use group casting; while increasing the cost for the players, it does mitigate the potentially devastating effects of massive damage, which protects the PCs because monsters don’t typically gain feats. Alternatively, a roll on the Wild Magic Surge table could prove disruptive, though I would remove most of the generally positive results and overweight some of the more negative results. Finally, I would recommend a scaling amount of psychic damage from the resulting magical backlash, triggered when the spell is completed. Something to the tune of 1d8 per spell level to each caster after the spell is completed could be a sufficient deterrent.

Finally, rather than multiplying damage dice, extending the range, duration, or area of effect of a spell might produce a similarly satisfying effect for group casting spells without the side effects of huge damage.

Thoughts and Feedback

Have you experimented with similar mechanics at your table, or have you attempted to implement these? We’d love to hear about it, either in the comments below, or via email.

2 responses to “Group Spellcasting: D&D 5E Homebrew Challenge

  1. That idea of combining spells has always been something I thought would be interesting!

    I remember playing Phantasy Star 4 (sega genesis) and Grandia Xtreme (ps2) which both had fun and rewarding ‘combo’ systems.

    Implementing some spell combinations that the group didn’t know about until they tried them could prove as either fun or frustrating, depending on the group. Personally, discovering these combos was one of the more fun parts of the aforementioned games!

  2. Looks pretty good. and Ready action really does make sense as a trigger. For me the big damage boost(and perhaps taking away of character niches) would be a squeaky issue since this slowly creeps into save or die and dice rolls would not really matter all that much for the bigger spells (I’m looking at you Disintegrate 25d6+40 (140 average using 5e rules) as shared casting with these rules, now that’s a poof!).

    I think with 5e walking the line between complicated and simplicity and often leaning towards the latter, let another caster help an ally as a reaction to their allies casting by making some sort of skill or ability check (Arcana) DC 10+ the spell’s final level. If successful the helper may expend equal the spell level slot from their daily usage and grants their ally the following: 1) if the spell can be cast at higher level, it is now cast at the next higher level if possible. 2) If the helper’s key caster ability is higher than their ally’s, the ally may use that value instead of their own for this spell’s attack roll or save DC. 3) any save against the spell is at disadvantage, and any spell attack has advantage. 4) if the spell has a duration of less one day, it now lasts for twice as long.

    One could roleplay this as “my arcana knowledge is so vast, that I sense the properties of your spell and boost them without really knowing how to truly cast it.” or better yet “hey we’ve been working together for while, of course I would pick up some of your mojo.”

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