Stealing boss ideas is easy once you get used to it. A DM can pick up almost any game with a boss, pick almost any boss in that game, and apply it to their game for a fun encounter. But, as I like to say, every project in the world, from D&D sessions to movies to entrepreneurial pursuits, begins with two steps:
- Hey, you know what would be cool?
- No, seriously, how would we even do that?
Every fun concept must eventually match up to reality. Once you have a boss idea, you have to apply it to the system rules (or choose which rules to violate/ignore, but that’s a whole other topic). That means figuring out how to work it into your campaign, but also balancing the encounter against what the party can handle.
Because the math for different editions of D&D varies so wildly, there isn’t a simple method for creating a boss. For example, I can’t say “an enemy with +2 AC has boss-level defenses”, and not just because that’s a boring way to do it. In 3E and Pathfinder, you can justify an arbitrary +2 AC bonus; just create a template that gives a +2 sacred (or insight, or whatever) bonus to AC and put it on that creature. But that +2 AC isn’t meaningful when the system is built for characters with wildly divergent attack bonuses, from the ridiculously accurate fighter to the rogue who really needs a flanking bonus to the wizard who only functions because he targets touch AC. +2 AC is a bigger deal in 4E, but AC is derived from a formula based on the enemy’s role and level. You can add +2 AC, but you need to create a tradeoff somewhere in accuracy or damage, and even then you’re playing fast and loose with the intent of the system. 5E also lets you give arbitrary bonuses, since the numbers that go into AC are also so hidden even the DM doesn’t know them, but the bonus itself is even more significant. A character is likely to gain less than +1 to his attack rolls every other level, so +2 AC is a hurdle it takes at least four levels to overcome. Any advice on numbers has to be system-specific.
But the process of balancing a boss goes deeper than just getting the correct numbers and watching the fight play out. There are general, system-agnostic steps for boss creation you should follow before you even think about the math involved. I like to think of them as a series of questions,
Figure out what you want to do: Why is this boss battle hard for the party? Should it challenge their battle strategy? Should it challenge their ability to think on their feet? Should it challenge their builds and luck?
The very first step in creating any boss fight (or campaign, or character, or house rule, etc.) is nailing down the concept. That concept should work its way through every other question. Here, the concept includes the boss’ role both in the campaign story and in game pacing. You have to decide whether the boss is a fallen angel or a robot snake, yes, but you also have to decide why the boss is scary, what you want the PCs to do (or avoid doing) to kill it, whether the terrain or arena has any effect, whether there’s a time limit, and so on. Until you know what you want to accomplish, you have no way to determining whether you’ve done it.
And if you don’t know how to make a boss scary, I have good news in the next post.
Know your players: Does your party enjoy showing off their build design skills through high-powered play? Do they prefer puzzle bosses who keep them guessing? If the battle comes down to wailing on the boss with the same moves over and over, is that a dramatic finish or a meaningless slog? And how do you feel about all this?
If the fight isn’t fun for you and your players, it’s wrong, and most players find it fun when they overcome a challenge. But different groups have different definitions of fun. Some prefer blowing enemies away with superior might and some prefer victory only when it comes at great personal sacrifice. Some prefer being challenged by having their powers fail and coming up with new strategies, and some prefer being challenged by overcoming an enemy’s numerically powerful defenses. Your concept has to work with what your players want from the game and vice versa. If it doesn’t, all the balancing in the world isn’t going to make the encounter fun.
Know your party: What’s the normal attack bonus of your group’s secondary damage-dealer? How many hit points does the healer have? What multi-target abilities can the group use more than once in a fight?
You don’t need to know everybody’s character sheet backward and forward, but you should have a sense for what the characters can do. You don’t want to give them a fight that’s too easy because you forgot the paladin’s smite allows it to ignore the dragon’s nigh-unbeatable DR, but you also don’t want a fight that’s too hard because only that paladin can damage it. You have to know where the party’s limits are before you can take them there. Remember, you’re not balancing a boss encounter against the rest of the system. You’re balancing it against the party you have right now, and nobody knows better than you what the party can and can’t handle.
Know your adventure: By the time the party reaches the boss, what have they done so far? How many fights have they had? When was their most recent long rest? Are they the type to fire all their big guns at the first sign of danger, or are they probably still hanging onto their nukes going into the fifth encounter?
Because a boss is a capstone, it generally comes at the end of something, and you have to consider everything that led to it. If the party fought through some dragons earlier in the dungeon, they might be low on healing because of all the breath weapons that deal automatic damage, which means you shouldn’t expect them to have their full healing compliment ready for the boss. But if the party cleric loves casting resist energy, the party might have withstood the breath weapons, so they have more healing but lack the ability to resist the boss’ cold aura. If the dragons had dozens of kobold allies, the wizard may be out of area-effect spells. If the dragons could sunder weapons, the rogue might be using his backup shortbow. You have full control over what the monsters on the way to the boss look like, so don’t accidentally make the players consume resources you think they’ll need for the boss. If you’re going to do that, do it intentionally.
Set your difficulty: Are you designing a miniboss who will harry the party but keep them healthy for the second half of the dungeon? Are you looking for something more serious to finish off a long dungeon? Or do you want to end an arc by putting the players’ backs against the wall and forcing them to win by the skin of their teeth?
Boss encounters are difficult, but they’re not all exactly as difficult as each other. Once you know what the boss is doing, when, and how, you have to decide how hard they do it. This step is so late in the list because you’re finally getting to the point where you put together numbers instead of loose concepts, and all your numbers should fit everything you’ve figured out so far. A boss who hits like a truck is fine, but difficulty is the difference between the boss hitting quite hard for its level, which means each blow is impressive and the players might have to spend some more time healing, and taking out a character with every swing, which means each blow is devastating and the players have to use their buffs, the arena, and clever tactics to avoid getting hit for the entire fight. Technically this is the first step in actual balancing. Everything else was just preparation, but if you’re not prepared to decide whether the boss’ spells slow, stun, paralyze, or kill the PCs, you’re as likely to make an ill-advised choice as a fun one.
Be ready to adjust: Say the rogue misses with every attack for six straight turns. Say the fighter lost her shield to a black pudding and without it she can’t use the four shield-based feats she has. Say the cleric didn’t prepare lesser restoration today even though he’s done it for every other session, which means he can’t heal the wizard’s Dexterity damage, or that the wizard obliviously prepared a dozen lightning bolts, which happens to be the perfect counter to the ability that makes the boss fight interesting. Does this change how you expected the boss battle to go? If it does, is there anything you need to do to make sure the fight is still fun?
You’re the DM. You’re well within your rights and powers to decide that the lich didn’t prepare shield of faith today so her AC is lower than normal, or that the demon has a real animosity toward dwarves so he’ll attack the defensive cleric instead of killing the wizard who can barely move. I’ve had plenty of monsters whose DR spontaneously dropped because the dungeon leading up to it was harder than I expected. It’s harder to justify making a boss more difficult because the players are doing too well, but you have every resource in the game at your fingertips if you need them. The time during (or right before) a boss fight is when you have the most information about how prepared the players and characters are for that fight, and you have both the opportunity and the ability to make changes.
With these answers you should have a handle on what makes your boss difficult, how difficult it is, and roughly how to plan to bring that difficulty for the table. For specific, actionable steps on bringing that difficulty about, you need to not only know how the boss will work with your game, your players, and your style; you also need the system knowledge to adjust the boss’ numbers and avoid the curse of the boring fight. Not everybody does. Luckily I do, via the experience gained by getting it wrong for several years, and I’ll cover that in a bit.