How does one homebrew a boss?

TW: corpses, body horror??

So, my players are facing a pretty big boss next session. It's an undead mass of corpses/flesh that slowly dissolves and assimilates with the people it eats. It's like a hivemind for a ton of ghouls in this cave system my players are in. I'd really like to make an original boss, something with a lot of tricks to it and strategy, but that's my most optimistic hope, since I'm not exactly the most experienced. Any advice? I've done bosses before, but for the most part I just took a statblock and renamed it, or just used a normal creature/npc.

Grab a stat block for the sort of critter you want, at more or less the right CR and combat style. Don't forget the minions when planning the encounter.

Give it legendary actions and saves.

Make sure it has enough HP that the party can't kill it too quickly. Defensive abilities, such as sacrificing minions to soak damage, reduce how tanky it needs to be. (And they're more flavorful.)

Replace or reflavor the attacks to fit the boss better, keeping the attack numbers and damage similar.

It sounds like this thing is sessile, so give it some kind of ability (probably one of its legendary actions) to get people out of its face.

Another one of its actions or legendary actions should probably be spawning a new ghoul.

Given that ghouls' signature ability is paralyzing people, a big mechanic of the fight could be the ghouls or the boss grabbing a paralyzed PC, bringing them over to the boss, and it starting to digest them. Take a look at the various critters with grapple or swallow mechanics to see how this could work.

Figure out at least one out for if you've overpowered the thing, and you're looking at a possible TPK. Be ready to kick it in before the PCs are well and truly screwed, so it's less obvious.

(It's actually more likely that you've underpowered it, and the players are going to nuke it faster than you expected. The fix for that is just "up the hit points on the fly", but also it's OK not to. Players feel good when they beat a scary enemy, and you can always take your lessons learned for future enemies.)

An important balancing concern is whether the players are going to come to this fight fresh or worn down by previous encounters.

Quote from CaptainJinxed >>

TW: corpses, body horror??

So, my players are facing a pretty big boss next session. It's an undead mass of corpses/flesh that slowly dissolves and assimilates with the people it eats. It's like a hivemind for a ton of ghouls in this cave system my players are in. I'd really like to make an original boss, something with a lot of tricks to it and strategy, but that's my most optimistic hope, since I'm not exactly the most experienced. Any advice? I've done bosses before, but for the most part I just took a statblock and renamed it, or just used a normal creature/npc.

From a rules perspective, a typical monster (up to CR 20) has

• HP: 15*(CR+1)
• Damage per round (ignoring hit probability): 6*(CR+1). Area effects count double.
• AC: 12 + CR/3
• Attack Bonus: 4 + CR/2

A monster that's higher in one area is typically lower in another, but there's a ton of noise in published stat blocks. A legendary monster should generally do about half of its damage budget on its turn, and the rest with legendary actions; other monsters typically do all their damage on their turn.

If you just follow those rules, you'll be similar in power to published monsters, so if you already know what CR of boss it takes to challenge your PCs (the DMG rules for that are, however, lousy) you can just design to the same benchmarks.

However, there's a separate issue: making a tactically interesting boss monster is a bit of an art, and published examples are not very good at it.

Some additional things to consider:

1. What is the action economy of your players? I.e, how many actions and bonus actions are they doing per round?  Your monsters should be equivilant to that, either through multi attack, additional monsters, or legendary actions.

2. How much health do your players have, and how much of that do you want to be able to take out at a given time?  I personally like to be able to deal at least 75% of a player’s max health in damage for a boss fight, forcing them to use healing resources.  Numbers might change based on party dynamic, particularly if they do not have a dedicated healer.

3. What can the boss do to change the battlefield?  A good boss is not static - they do things that might change the combat math and force folks to think on their toes. 

Quote from Caerwyn_Glyndwr >>

1. What is the action economy of your players? I.e, how many actions and bonus actions are they doing per round?  Your monsters should be equivilant to that, either through multi attack, additional monsters, or legendary actions.

This really isn't true. Actual number of actions only matters when there are abilities in play that add or subtract actions (if monster A has one action that does 100 damage, and monster B has two actions that do 50 damage, they both do 100 damage normally, but removing or adding an action from monster A matters a lot more than for monster B), otherwise all that matters is the effect of those actions.

Quote from Caerwyn_Glyndwr >>

2. How much health do your players have, and how much of that do you want to be able to take out at a given time?  I personally like to be able to deal at least 75% of a player’s max health in damage for a boss fight, forcing them to use healing resources.

That's definitely an issue of what your players actually consider challenging. I generally prefer to drop multiple PCs to zero hit points.

Quote from Pantagruel666 >>

Quote from Caerwyn_Glyndwr >>

1. What is the action economy of your players? I.e, how many actions and bonus actions are they doing per round?  Your monsters should be equivilant to that, either through multi attack, additional monsters, or legendary actions.

This really isn't true. Actual number of actions only matters when there are abilities in play that add or subtract actions (if monster A has one action that does 100 damage, and monster B has two actions that do 50 damage, they both do 100 damage normally, but removing or adding an action from monster A matters a lot more than for monster B), otherwise all that matters is the effect of those actions.

Quote from Caerwyn_Glyndwr >>

2. How much health do your players have, and how much of that do you want to be able to take out at a given time?  I personally like to be able to deal at least 75% of a player’s max health in damage for a boss fight, forcing them to use healing resources.

That's definitely an issue of what your players actually consider challenging. I generally prefer to drop multiple PCs to zero hit points.

 
Offering clarification due to bad writing (yay, head colds!) on both points: 

For the first point, I agree there are often better ways for balance.  But looking at action economy is a very easy, concrete way to judge a party’s capabilities versus the boss, particularly when you are evaluating in light of things like generic attacks, which, even on high level monsters, are fairly low-powered in the individual.  If using other monsters as a comparison or starting point, this is a good data point to keep in mind.

For the second point, I left out a pretty important part - the 75% damage was in a single round.  Thus, while not likely to kill a player in round 1 (which is not great design since you might take them out before a response can be performed), you’re very likely to take someone out during a given boss fight, or at least put enough pressure that the party has to work in order to avoid that death. 

My party is made up of three level fours; a paladin, a fighter, and a rogue. The paladin has 40 HP, the fighter has 42 HP, and the rogue has 39 HP. However, said rogue is also an idiot and decided to get drunk before the fight and, additionally, is on 29 HP at the moment due to trying to rob someone earlier.

One thing you can always do is give it 'HP features' - abilities that go off when the monster hits a certain HP threshold (like say rage). This allows you to adapt the monster mid-combat without fudging things if it's proving too weak. On the flip side, you can ignore (or delay) that feature (because they don't know it's there) if it's proving too strong. 

For example, I created Mr. X from the Resident Evil games. He was immune to low level attacks & magic so my players had to be able to run away from him. However, when they got strong enough to kill him, I knew his slower speed would make him an easy target so when he took damage, his speed doubled, he raged & he got a misty step equivalent bonus action.

Some random suggestions for this specific monster:

• Hurl a glob of flesh that does a lot of damage but eats away at its own HP (and potentially leaves it a vulnerable striking point that will allow the players to do more damage) -> if they're perceptive they'll wonder why it's not using this powerful attack more often.
• Some kind of restraining feature - first it grapples you, then next turn if you don't get out it sucks you inside & starts dissolving your flesh (maybe it also heals while you're in there)
• Clayface like malleable flesh allowing it to do different kind of attacks - a restraining whip, a wounding scythe that causes an acid wound until you get magic healing and/or use an action to make a medicine check, a heavy hammer that could knock them prone.
• A stash of corpses that it stores within it that it consumes to heal every you time drop it 0 - so you have to go inside and destroy the corpses somehow - maybe the hurl glob feature reveals a corpse until the start of its next turn - if you pull all the corpses out with successful athletic checks [or whatever cool idea they can come up with] it will finally die (but if you're doing this, I'd suggest making it low HP).
• They don't seem very wise based on their classes so you could do a sanctuary spell sort of feature and they have to come up with inventive ways of hurting it, like say maybe there are stalactites that are loose that you could knock down or cause rockfall or shove it down that gaping pit that's conveniently right behind it - none of these are targeted attacks or harmful spells -

The caveat I put on all this, however, is you have a low level party with limited action economy and not much healing (unless you've given them a bunch of healing potions). I'm not sure if you can make a fight last long enough to apply many of these suggestions.

Some other points of consideration:

• I like to reflavour features like paralysis when used by an enemies so they don't waste an entire turn sitting there - e.g. Paralysis – auto fail concentration checks, movement 0, attacks against you have adv, your atks have dis, melee atks auto crit and if you cast a spell you have to succeed on a con save (DC = 10 + spell level) or fail to cast the spell (but keep the spell slot) -> they can still do things but they're more limited.
• If they've magic items, that changes the equation again.
• I'm not sure they're strong or numerous enough to face a boss + minions (I say with the caveat of I DM large groups of 6 or more players so I'm not used to running low level fights with low numbers)
• If you've time and inclination, you can always run a simulation of the fight yourself - roll for the monster, roll for your players & see how they do (I find that more fun than doing lots of math, even it probably takes longer).
• You can edit it on the fly - I know some people don't like to do that but I do find sometimes I seriously underestimate or overestimate my players.

As a general point, what I've found with homebrew monsters is it's a lot of trial and error. I've been doing it quite a while and I've developed a kind of instinct. I also find it's easier at higher levels as get more familiar with what my players can do. Like I've one player who with action surge and other abilities gets 8 attacks in one turn and does like 30-35 damage on a hit so I knew my boss with only 200 odd HP would need some way to protect itself from him so I gave it an equivalent to the Warlock Tomb of Levisticus feature. As was previously said though, you'll probably underestimate your players rather than overestimate - that happened a lot in my early homebrew days and still does sometimes. I do find the best thing I can do is just look at my past experiences with them - how much damage do they do, how often do I hit them usually, how good are they at being inventive, what magic items do they and I go from there.

Sorry for the long post, but I hope this helps.

This is AMAZING, thank you SO MUCH!!!