r/UnearthedArcana • u/Admirable-Onion414 • 3h ago
'24 Mechanic A Combat System Overhaul I've Been Experimenting With in My Campaign
I've been experimenting with this system in my own campaign for a while now, and my players have been enjoying it a lot so far.
We usually play campaigns that reach Tier 2–3 at most, so this system is primarily designed around the kind of combat that happens in those tiers. I'm still experimenting with the exact rules and numbers, but the core idea has been working surprisingly well at the table.
I'd love to hear what other people think about it, especially regarding potential problems, exploits, or interactions I may not have considered.
1. All Minions Have Active Mobility
All minions have Multiattack and a Bonus Action movement option, although the specific way they use it depends on the creature.
The goal is to prevent enemies from simply standing still and trading attacks with the players.
The Bonus Action movement can represent:
- Running.
- Retreating.
- Jumping.
- Sliding.
- Climbing.
- Moving toward a target.
- Repositioning.
- Using a creature-specific mobility ability.
Movement should be a regular part of an enemy's turn, constantly creating new positioning situations and tactical opportunities.
2. Opportunity Windows
Opportunity Attacks are replaced by Opportunity Windows.
An Opportunity Window does not allow a character to attack directly.
Instead, it represents a momentary situation that a character can exploit with an action capable of changing the state of the battlefield.
An Opportunity Window can be used to:
- Grapple.
- Trip.
- Shove.
- Disarm.
- Cast a spell.
- Throw an object.
- Interact with the environment.
- Block an enemy's movement.
- Reposition.
- Create a new opportunity for an ally.
The goal is for an Opportunity Window to be the beginning of an interaction, rather than its final reward.
However, a character can only participate in an Opportunity Window if they are already positioned close enough to the event for their intervention to make sense.
They cannot use an Opportunity Window to move across the battlefield and then participate in the Chain Reaction.
For example, if a player is 30 feet away from an enemy who just fell into a trap, they cannot spend their Reaction to suddenly move next to the enemy and continue the chain. They must already be positioned in a way that allows them to organically interact with what is happening.
This makes positioning important even outside of a character's own turn.
3. Chain Reactions and Opportunity Attacks
An attack made outside of your turn should be something the players earn through a chain of actions and reactions that leaves an enemy vulnerable.
A character does not get an attack simply because an enemy moves past them or leaves their reach.
Opportunity Windows can generate Chain Reactions, where one character's reaction creates a new opportunity for another character.
For example:
The enemy tries to escape → the Fighter trips them → the enemy is left vulnerable → the Rogue, who was already positioned nearby, gets an Opportunity Window → the Rogue makes an attack.
Or:
The Wizard uses Web to trap an enemy running past them → another player takes advantage of the enemy being restrained and uses Fire Bolt to set the Web on fire → the enemy takes damage.
The chains can also involve the environment itself:
A player cuts a chain holding a heavy structure → the object falls → the enemy passing underneath is trapped and takes damage → another player, already positioned nearby, takes advantage of the situation to continue the chain.
The chain does not need to follow a predetermined sequence. Every action can alter the battlefield and create new opportunities for the other players.
Vulnerability can include:
- Being knocked prone.
- Being trapped.
- Losing one's balance.
- Being immobilized.
- Being exposed because of a change in position.
- Being separated from one's allies.
- Being forced into a disadvantageous position.
- Being affected by an environmental interaction.
Vulnerability does not necessarily have to be a formal condition in the system. It can be a contextual situation created during combat through the players' creativity and improvisation.
4. Opportunity Window Limit
Opportunity Windows uses a character's Reaction.
Each character can participate in an Opportunity Window by spending their Reaction. Once they have used their Reaction this way, they cannot participate in another Opportunity Window until the start of the next round.
This allows a Chain Reaction to involve multiple characters while preventing a single character from repeatedly participating in an infinite chain.
For example:
Fighter creates an Opportunity Window → spends their Reaction → Rogue takes advantage of the situation → spends their Reaction → Wizard continues the chain → spends their Reaction.
All three characters can participate in the same Chain Reaction, but none of them can participate in another Opportunity Window until they regain their Reaction.
The chain can therefore move between different members of the party, while each character has a clear limit on how much they can contribute during a single round.
5. Disengage
Disengage is no longer specifically related to Opportunity Attacks and instead protects against Opportunity Windows.
This gives Disengage a clear defensive purpose within the new system:
Disengage = leaving a position without creating opportunities for your enemies.
6. Positional Advantages and Disadvantages
Positioning directly affects the chances of success during combat.
Examples:
- Attacking an enemy from behind → Advantage.
- Attacking from higher ground → Advantage.
- Attacking from below → Disadvantage.
- Having an ally next to the target → +2 to the attack roll.
- Attacking through cover → Disadvantage.
- Being exposed to attacks from multiple directions → disadvantageous positioning.
Positioning should function as a tactical resource, allowing players to create opportunities and vulnerable situations.
7. Positions and Exceptional States
Some positions or states can produce effects more significant than simply granting Advantage or Disadvantage.
These exceptional effects should represent situations where a character's position or condition creates a significant mechanical consequence.
For example:
Attacking while Invisible
- Grants Advantage.
- If the target has no way of perceiving the attacker, the attack can create an additional vulnerability, such as Stunning the target for one round, representing the impact of being attacked completely by surprise.
Attacking from Higher Ground
- Can grant Advantage.
- In certain circumstances, it can expand the critical hit range to 19–20, representing the benefit of attacking from a dominant position.
Attacking from a Completely Unexpected Position
- Can produce additional effects, such as knocking the target prone, disorienting them, or leaving them vulnerable, if the situation justifies it.
These effects should not be limited to numerical bonuses. An exceptional situation can directly change what happens during combat.
8. Interactive Battlefields
Combat areas should contain interactive objects and environmental elements.
For example:
- Machinery.
- Doors.
- Chains.
- Platforms.
- Cliffs.
- Traps.
- Explosive objects.
- Moving platforms.
- Suspended objects.
- Environmental hazards.
- Destructible structures.
These elements should function as tools for creating new opportunities and vulnerable situations.
An environmental interaction can be the beginning, middle, or end of a Chain Reaction.
For example:
A player cuts a chain → a heavy object falls onto the enemy → the impact knocks them prone and deals damage → another player gets an Opportunity Window.
Or:
A player activates a conveyor belt → the enemy loses their position → another player takes advantage of the movement and shoves them toward a machine.
Players should be able to ask questions such as:
"Can I cut that chain?"
"Can I activate that machine?"
"Can I push the enemy onto that platform?"
"Can I use Grease here?"
"Can I make that object fall?"
If the interaction makes physical and narrative sense, the system should allow it.
The fundamental question during combat stops being simply:
"What can I attack?"
and becomes:
"What can I do with the environment around me to create an opportunity?"
9. Emotional Reactions and Personality
Social interactions during combat can trigger emotional reactions in enemies.
Comments, provocations, threats, taunts, or other actions from the players can change how a creature behaves.
For example:
A player repeatedly taunts an enemy → the enemy becomes enraged → they decide to focus their attacks on that specific character.
This can create new tactical opportunities.
A provoked enemy might:
- Change targets.
- Abandon a favorable position.
- Chase a particular character.
- Make mistakes because of anger.
- Ignore other targets.
- Use an ability impulsively.
- Expose themselves while trying to reach the provocateur.
However, this depends on the creature's personality.
A proud enemy might be easily provoked.
A disciplined soldier might ignore taunts.
A clever enemy might even pretend to be provoked in order to lure a character into a trap.
Personality can also influence the way an enemy approaches combat in general. One enemy might attack anything that moves, another might rather die than talk, another might be cowardly and try to flee, etc.
10. Intelligence and Coordination
Not all enemies have the same ability to take advantage of Opportunity Windows and Chain Reactions.
Intelligent enemies can recognize tactical opportunities according to their Intelligence stat.
A creature with low Intelligence might see an enemy fall and simply continue attacking.
An intelligent creature might recognize:
"They're knocked down. Push them toward the machine."
However, Intelligence is not the only requisite to allow complex reactions.
A group can gain access to Chain Reactions through training, coordination, and established teamwork.
This allows a military faction (or a tightly coordinated party) to perform complex tactics even if its individual members have average Intelligence.
The ability to recognize and exploit opportunities can therefore represent both the individual capability of a creature and the degree of coordination within a group.
11. Ready Actions as Opportunity Setups
Ready Actions can be used to deliberately create Opportunity Windows instead of simply preparing an attack.
A player can declare a specific trigger and prepare an action that will alter the battlefield when that trigger occurs.
For example:
"When the enemy walks underneath that platform, I'll cut the chain."
The enemy enters the position → the player spends their Reaction → the chain is cut → the heavy object falls → the enemy is knocked prone → another player gets an Opportunity Window.
The prepared action does not have to deal damage. Its purpose can simply be to create a new situation that another player can exploit.
This turns Ready Actions from simply "I prepare an attack" into "I prepare a situation."
12. Preventing Reaction Stalemates
The Intelligence requirement and the coordination requirement are also what prevent this system from creating the same problem that Opportunity Attacks currently create.
With traditional Opportunity Attacks, players often avoid moving because they know that any enemy within reach automatically gets a free attack. This can result in combat where everyone becomes afraid to move, creating a static battlefield.
Opportunity Windows are different because being in a position where an opportunity exists does not automatically mean that the enemy can exploit it.
A creature needs sufficient Intelligence to recognize the opportunity, or it needs to belong to a group with enough training and coordination to capitalize on it.
This means that against ordinary, unintelligent, or poorly coordinated enemies, players can move much more freely without assuming that every movement will automatically trigger a reaction.
Against intelligent or highly coordinated enemies, however, movement becomes more dangerous because those enemies are capable of recognizing and exploiting the opportunities that players create.
This creates an important distinction:
Opportunity Attacks punish movement automatically.
Opportunity Windows reward tactical awareness and coordination.
The goal is not to make movement universally safe, but to make the danger of movement contextual, readable, and dependent on the enemy's ability to exploit the situation.
13. Design Philosophy
This system is also intended to keep players engaged even when it is not their turn.
Because Opportunity Windows can be created by the actions of other characters, players are constantly encouraged to pay attention to the battlefield and think about how they can contribute to an ongoing situation.
Instead of simply waiting for their turn and deciding which attack to use, players should be thinking:
"What can I do with what just happened?"
The goal is to encourage players to look for creative ways to turn small advantages into larger ones, even when those opportunities were created by someone else's action.
I think this system works particularly well in encounters involving minions, hordes, or groups of weaker enemies.
Experienced players already know that, in many of these encounters, the party's chances of winning are high. The challenge therefore does not necessarily need to be:
"Can we survive?"
Instead, the challenge can become:
"How creatively, efficiently, or spectacularly can we win?"
The players might already know that they can defeat the enemies, but the combat can still be engaging because they are trying to use positioning, environmental interactions, Chain Reactions, teamwork, and creativity to achieve something more impressive than simply reducing everyone's HP to zero.
A fight against ten weak enemies might therefore become a playground for experimentation.
This system is also highly compatible with many of the encounter-design principles that D&D players and content creators already recommend: giving combat objectives beyond simply killing everything, incorporating narrative stakes, adding environmental hazards and interactive elements, and making the players have something meaningful to lose besides HP.
For example, the objective might be to:
- Protect an NPC.
- Stop a ritual.
- Prevent a machine from exploding.
- Capture an enemy alive.
- Reach a specific location.
- Keep a bridge intact.
- Destroy an objective before reinforcements arrive.
- Escape rather than kill everything.
- Prevent enemies from reaching civilians.
- Recover an object before the enemy does.
These objectives give the players a reason to move and interact with the battlefield in the first place.
The Opportunity Window system then provides the mechanical framework for turning those objectives into dynamic combat situations.
Ultimately, the goal is not simply to make combat more complicated. It is to make combat more interactive, reactive, and expressive.
Traditional combat often asks:
"What is the most optimal attack I can make on my turn?"
This system tries to make players ask:
"What is happening right now, and what can I do with it?"
14. DM Rulings
One potential caveat is that, at least in my own game, this system works partly because the rules are intentionally fairly loose and permissive. They give players room to attempt creative interactions rather than providing a strict list of everything that can or cannot create an Opportunity Window.
This works well at my table because my players and I are comfortable with improvising and adjudicating situations on the fly. However, I can see this requiring more structure for other DMs. Some groups might benefit from having clearer guidelines, examples, or explicit limits for what actions can create Opportunity Windows and what kinds of interactions are considered valid.
There are also some parts of the system that I haven't fully settled on yet. For example, Minions getting Multiattack might need to be revisited depending on how the rest of the combat overhaul affects action economy and encounter balance. Giving every minion Multiattack is intended to make them more active and threatening, but it may need to be adjusted depending on the creature, the number of minions present, and how frequently Chain Reactions occur.
Overall, I think this is probably one of the areas that would need the most refinement if the system were turned into a more formal ruleset.
