The big solo Monster fight is an iconic part of DnD. Yet much of the advice about creating solo encounters is a curt "Don't" or ruminations about the minutiae of the "Action Economy".
I find this odd because in many ways solo monsters encounters are easier to construct than multi-enemy encounters. The calculations are linear instead of non-linear. Running them can also be easier because there is only one Monster you have to think about. The DM doesn't have to look back and forth between different stat blocks.
There are a few common pitfalls DMs can fall into when creating Solo encounters. With the quick steps below you can ensure your next solo encounter avoids these missteps and leaves an indelible mark on the players.
Step 0: The idea
Good solo monsters are often built around one-of-a-kind evocative ideas that connect their lore to the themes of the campaign. I am assuming you have an exciting idea for the solo monster. If not, the following steps won't help. Tweaking numbers won't make a flat idea fun. It will just be putting lipstick on a pig.
Step 1: Open Xanathars
Xanathar's Guide to Everything has a table in the encounter building section called "Solo Monster Challenge Rating". You input your party size and level. The table spits out a CR to use. Add +2 to that CR for a perilous battle!
Once you have the target CR look at the published monsters for creatures with that CR. Consider Legendary monsters first. If you want to use a non-Legendary monster I will go over what adjustments are needed below.
If you're using a published Legendary monster you are done! The remaining steps are optional. They will help you fine tune the encounter but if you're short on prep time then you can go with what you've got.
Step 2: Estimate the party damage
Calculate the amount of damage the party can do each turn. You can just take their average attack damage and acknowledge that the real value will be slightly lower because some attacks will miss. Or you can use the Monster's AC and Saves to more accurately measure the damage the party will do.
Divide the Monster's HP by the party damage to estimate how many turns the Monster will survive. Does that number feels right? A common pitfall is giving solo Monsters too little HP. Resulting in an anti-climatic battle where the dies in 2 turns without doing anything.
Legendary Resistances are critical for every solo Monster. There are multiple must-pass effects that turn off the whole encounter if they occur. Most effects that incapacitate (like Stun or Paralyze) are must-pass. Legendary Resistances are what prevent the encounter from ending on one poor roll on the first turn.
Look at what must-pass effects the party could muster against the Monster. How many of those effects could the party throw at the Monster during the turns it is alive? If that number greatly exceeds the (usually 3) Legendary Resistances then be aware that the players may go for a Legendary Resistance strategy rather than a HP strategy.
Step 3: Estimate Monster's damage
Calculate the amount of damage the Monster can do each turn. Multiply that by how many turns you expect the Monster to survive (from step 2). The closer that the total Monster damage is to the sum of the party's HP, the more challenging the fight. If the total Monster damage is greater than the total HP of the party the Monster may be too powerful. A common pitfall is the Monster is so understrength that it barely does half the party's HP in damage before it dies. Meaning the players will never be worried.
In general, you want to avoid solo monsters that dump all their damage on a single PC. Be careful with any solo monster that can drop a PC in a single turn. Such monsters can make the encounter too deadly. They can also produce feel bad moments where the focused player blames the party for not doing more to save them, even if the monster never gave the party the opportunity.
Prefer AoE attacks instead. That way all members of the party feel some danger. Or use "Eye of Sauron" effects (aka effects that PCs become immune to after succeeding once). So that each turn a new player finds themselves in the hot seat as the powerful effect is directed toward them.
Legendary Actions: If you want a solo monster to do ~40 damage per turn you could have the monster take one action per turn (like a monster without Legendary Actions). Or you could give the monster Legendary Actions that each do ~10 damage per action. There are a bunch of subtle design reasons to prefer lots of small actions to one big action:
- Smaller actions are less swingy. If the monster gets a Crit on its one big action it will almost certainly drop a PC.
- Smaller actions give players more moments of interaction with the monster.
- Players won't take their actions consecutively. This makes it easier for players to figure out what the best combination of steps are (smaller search space). It also reduces the incentive for one player to quarterback another player.
- Multiple actions gives the monster more ways to self synergize. One of their Legendary actions can set up their main action. Smart players may be able to disrupt these synergies.
- If the monster only takes one big action that action needs to be very strong. Much stronger than casual intuition would indicate. While you can scale up attacks easily, spells are much harder to scale up. Having the solo monster cast 4 spells via Legendary Actions will be closer to what you want then trying to make a single super spell work.
(Optional) Step 4: Refine estimates
The raw party damage per turn, monster damage per turn, party HP, and monster HP are a good heuristic. But sometimes the monster or party has unique capabilities that have non-linear impacts. If you have some extra time, give some considerations to those.
You can also consider what tactics the party or Monster will use. The "Design Patterns" section below talks about some common tactical archetypes.
Adjusting non-Legendary Monsters
If you are using a non-Legendary Monster you will want to give it Legendary Resistances and Legendary Actions. Legendary Resistances are usually 3 per day. For Legendary Actions, you will want to divide its damage output into the Monster's main action and 3 Legendary Actions.
The Legendary Actions should be inspired by the theme of the Monster. Look at the existing Legendary Monsters if you want more examples of things you could do. Try and have interesting combos the actions can create.
In general, you want to avoid solo monsters that dump all their damage on a single PC. Prefer AoE attacks instead. That way all members of the party feel some danger. Or use "Eye of Sauron" effects (aka effects that PCs become immune to after succeeding once). So that each turn a new player finds themselves in the hot seat as the powerful effect is directed toward them.
Solo Monster Design Patterns:
There are some broad archetypes that solo monsters will tend to fall into:
Big Hello: These are monsters with a powerful AoE attack that they use on the first turn. Most Dragons fall into this category.
This initial attack shouldn't drop the party, but should put them on the back foot. Weaker classes may be so injured that they can't consistently keep Concentration spells going. Other characters have to split their attention between going after the monster and keeping PCs alive.
It is critical to communicate to the players that the Monster's initial attack can't be repeated on the second turn. After the "Big Hello" the players will be trying to determine if this is a fight they can take. If they believe the attack is representative of what the Monster can do each turn, they will conclude running is the only option. If you want the party to stick it out, you have to tell them it isn't representative.
Eye of Sauron: These Monsters have powerful single target effects that they can use each turn. But once a PC makes a save against the effect they are immune. Each turn the Monster blasts another PC.
The Yeti's Chilling Gaze, the spell Eyebite, and the spell Feeblemind are all examples. The PC being blasted is in danger but if they can survive then they will get some breathing room while the Monster blasts someone else.
Boring Brute: These are solo Monsters that only have melee attacks. They walk up to the party and everyone rolls dice at each other until they die. It is easy for a solo Monster to become this if the DM is inattentive with their design. Give the Monster ways to harm PCs not engaged in melee with it. Or have it charge back line characters forcing the players to chase it.
Skirmisher: Some Monsters lack the raw HP to survive but instead have Flying, Teleport or high mobility plus a ranged attack. These Monsters challenge the party to find a way to engage them. If the PCs can close the gap they can bring the Monster down (relatively) quickly. Flying Monsters like the Dragons fall into this category.
Melee Death Machine: This archetype is the reverse Skirmisher. It charges the PCs and they have to try and stay away from it. Unlike the "Boring Brute" trading blows isn't an effective strategy. The party has to whittle it down while avoiding its limited attacks.
This style of Monster can be very frustrating to melee focused characters. Like the Boring Brute, it can cause problems if the DM unintentionally makes the Monster this way.
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