Spider Broodmother: Design with Me
CW: Discussion of Spiders, Discussion of Body Horror
This post is a part of an ongoing Upgraded Monsters series. Click the link to check out all posts in the series.
I'm hoping to expand beyond just showing links to work that I create. For this sort of article, I'm going to walk through my process of designing a small piece of homebrew material, so that you can see my creative progress – and hopefully have it inspire and teach you about how you can go about creating your own! This is not really in keeping with the "history-influenced D&D" theme of the blog, but I think breaking down my monster-creation process can still be a useful and relevant resource for future design-with-me articles that are more focused on history.
The finished product from this article is available on DMsGuild as a Pay-What-You-Want title.
In this Design With Me, we’ll be making a homebrew version of a spider for a D&D 5e campaign. I hope it gives you some insight into the monster creation and balancing process. And it means we're taking a break from all the Western-heavy homebrew since the blog's inception.
This article may include affiliate links.
Inspiration
The first thing to do when creating homebrew is to get an idea of what you're hoping to create.
For this project, one of my main gripes with monsters in D&D 5e is that they rarely have interesting tactical decisions to make. There's little variation in terms of what a creature can do except try to hit the party as hard as they can. This is something that 4e tried to address with monster roles – titles like "artillery", "controller", and "skirmisher" tried to tell GMs how to run a monster. Do they try to stay in the back? Do they attack wildly? Do they have abilities that let them disengage from combat and maneuver to somewhere more easily?
A ton of work towards alleviating this common complaint has been done by Keith Ammann, whose award-winning blog The Monsters Know What They're Doing focuses on developing tactical decisions for various monster manual creatures. Seriously, Ammann's work is incredible and very, very valuable to me as a GM, and I'll frequently use his articles when trying to plan interesting combats. If you haven't checked out his work, you should do so. However, his work focuses on creating tactical decisions for monsters using their existing abilities; I want more interesting abilities to be available in the stat blocks. Plus, he has not tackled a lot of common beasts, including giant spiders.
My main source of inspiration for this creation is the giant spiders in the video game, Divinity 2: Original Sin. The Divinity games are created by Larian Studios, the studio behind the 2023 Game-of-the-Year, Baldur's Gate 3. One type of spider enemy in Divinity 2 is able to lay an egg in you as an attack, and a turn later, a weaker spider crawls out of you to attack. It's a horrifying and effectively frightening monster. Beyond appealing to some body horror that I think makes these creatures frightening from a purely narrative sense (after all, what is grosser than a spider crawling out of your body? That's nightmare fuel), it also gives the players an interesting tactical goal: focusing fire on these spiders because they're able to spawn more enemies. In my play-through of Divinity, taking down these spiders became a goal even above killing potentially more damaging or dangerous enemies to avoid being swarmed by more spiders. Choosing which target to prioritize creates an actual decision point for players.
General Power
I tend to hate D&D's CR system. The math is not great overall, and it makes it difficult to balance encounters with a lot of creatures – which is a better method of balancing to preserve the enemies and the PCs having a similar number of actions. I prefer the system developed by the AngryGM, one of my go-to GM advice blogs.
"Angry" builds a system that lumps players into level tiers and then classifies monsters by roughly how many of them should appear in a set against the average party. In most of my homebrew for my personal games, I use this system entirely instead of CR; for things that I'm publishing, I'll go back and figure out what the CR is and list that, but I'll use Angry's guidelines while I'm actually doing the monster building. It is so much simpler to use.
So, using the table that Angry lays out in his article, I want our spider-implantation creature to be Journeyman tier – appropriate for levels 3-5. Higher than that, my players would be unlikely to encounter simply 'normal spiders' in any way that is meant to be significantly challenging; they're above that, unless I want a smaller challenge or a whole swarm of enemies. Lower than that, and my creature won't get much use since I try to avoid spending much time at levels 1 and 2.
I also think I should aim for a "Pair" creature. I don't want my spider to be encountered alone; remember, part of the inspiration is that it can be partnered with a different target to create a tactical decision of whether to go after the 'weaker' spider that spawns more spiders or to go after a more outright dangerous enemy. This decision point wouldn't work if I made it a solo creature. On the other hand, if it is a "Party" creature, I'd be worried about my players finishing it off too quickly before its signature ability has a chance to creep out my party. It needs to have some decent survivability.
Spider Traits
Spiders in the D&D 5e basic rules, including the Phase Spider, the Spider, and the Giant Spider, all have 2 common traits. When building out a new monster of the same general type as an existing one, including these common traits is essential. D&D uses common traits to establish some sort of mechanical unity across a creature type – in this case, spiders. Our two common spider traits are:
Spider Climb. The spider can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Web Walker. The spider ignores movement restrictions caused by webbing.
Signature Ability
We want to make sure that our signature ability--the concept around which we're designing the creature--is one of the first things we build so that everything else can complement that ability.
Our spider – which I'm calling the "Broodmother" because of its ability to spawn additional spiders – should have some sort of way of implanting a spider spawn in a PC. I know that we should have some sort of attack roll or saving throw so that there is a chance that the ability fails; it's one of the cardinal rules of D&D that all things, including most spells, should have a chance to fail.
Second, I don't want this ability to be usable every turn. I think it is more impactful if the Broodmother needs to gear up for it somewhat, making it clear that this ability is a threat. In practice, we can even use the mechanic described in the introduction of this post by the AngryGM to telegraph that the Broodmother is ready for her parasitic attack so that players can try to work around it and prepare for it:
To limit an ability's use in D&D, the easiest mechanic is to put a cap on how many times it can be used (for example, once per day). This is boring! The Broodmother would simply pop out as many spawns as she's able to at the beginning of the fight, and then be a more dull spider the rest of the time that she survives. That doesn't keep our players on our toes or keep the decision interesting throughout the combat.
Instead, I'm going to borrow what only a few other monsters do, and which was way more prevalent in D&D 4e: recharge. The way recharge abilities work is that, at the end of each turn, the monster rolls 1d6; if the recharge number comes up, they can reuse the ability. I try to only have one "recharge" ability per monster–or really, per combat--to make bookkeeping easier so that I'm not tracking a bunch of different d6 rolls each round, but that's not a hard-and-fast rule. Most recharge abilities are on a 5 or 6 so that they'll be usable about 1/3 of rounds. Rolling for recharge at the end of the round, instead of the beginning, also gives us the ability to use the AngryGM "telegraphing"/"barking"/"charge up" ability that he describes.
Here's the working form of the ability, with no specific numbers attached to it.
Parasitic Implantation (Recharge 5-6). Melee Weapon Attack: +X to hit, range 5 ft, one target. Hit XdX+X piercing damage. At the end of the target's next turn, unless the target gains hit points before then, the target takes an additional XdX piercing damage and a Spider Broodling emerges into an unoccupied space adjacent to them. The broodling enters the initiative order immediately after it emerges.
(Yes, this does mean I'll need to repeat this process for a Spider Broodling; I'm not going to show you all that process, but the Broodling stat block will be in the final product. I'm choosing to do that instead of making a normal spider or a normal giant spider because neither of those really fits the right level of danger for what I want to spawn).
Basic Attack
If we were to assign a 4e-style monster role to our creature, I'd lean towards artillery. Yes, the parasitic implantation attack requires adjacency, but I also don't want the broodmother to be right smack dab in the middle of combat. Going back to our tactical decision-making idea, that is more of a choice if the party would have to peel off to attack it, diverting attention from the center of the battlefield where other targets will be. Plus, staying at range means that our creature will probably survive for longer. That said, I don't want the range to be too large – the creature needs to be able to dart in for a parasitic implantation attack in a single turn.
That means our bread-and-butter attack that will make up the bulk of the creature's turns should be a ranged attack. When I think of spiders, I think of venom, so when building a ranged attack, I imagine a spider that spits venom. There are real-world examples of this in snakes, but for a giant spider, I don't think it is a particularly absurd concept to steal from a different venomous creature. Our attack can do poison damage.
Venomous Spit. Ranged Weapon Attack: +X to hit, range 20/40 ft, one target. Hit: XdX+X poison damage.
Math
Now for the boring part – balance and numbers.
I want our creature to be a little more defensive than offensive for the reasons we've already discussed: I want the creature to have some survivability, and in a fight, I want the damage output of the Broodmother to be overshadowed by a partner creature, to make the tactical choice of "immediate high-damage threat, or Broodmother that spawns extra creatures" a more difficult decision.
According to the chart we took from the AngryGM, our Journeyman Pair creature needs to deal 16 damage per round to be at a -1 strength for damage (to keep the offense a little weak). Getting up to that number just on our venomous spit would be difficult (like 3d10 – a major attack), so let's give our creature multiattack, the 5e Monster Manual trick to increase damage numbers. If our spider makes two venomous spit attacks each round, each needs to deal 8 damage.
Ok, that's much easier to get to – 2d6+1 averages to 8. When calculating the expected value from dice, you always divide the die size by 2 and add 0.5 (to account for the minimum roll being 1). So 1d6 has an average value of 3.5, for example. We could also do 1d12+1, but that both makes the attack a lot more variable in the outcome and technically averages 7.5, rather than 8 – not a huge difference, but if we prefer the more standard outcome of 2d6 anyways, let's stick with that.
Now, where is our +1 coming from? The spit damage doesn't feel like it should be based on Strength or Dexterity like damage normally is; it doesn't matter how strong the spider is to determine how poisonous it is. However, I also think it shouldn't be a mental attribute. Let's use CON – it's not a perfect fit, but CON is often used around poison resistance, so I think it does work to make it fit as the potency of the spider's venom.
With a +1 to CON, let's go ahead and figure out our spider's HP. We're aiming for somewhere around 68 HP. I want our giant spider to be a medium creature – I think a large creature is too big, even a giant spider shouldn't be significantly larger than a human; but it has to be big enough that it cannot simply be squished. Medium monsters always have a hit die of a d8, so 1d8+1 (from CON) gives us an average of 5.5 HP per 'level'. Monsters don't actually level up in the same way as characters, so that's just a shorthand. To get 68 HP, we need 12.36 levels. Let's round that to 12 levels, which gives us 66 HP. Close enough – 2 HP isn't going to make a difference.
But wait! What about our parasitic implantation attack? Well, that is a little harder to balance – how much 'extra' does the spawned creature count as? Here's where D&D being not actually all that carefully balanced helps us out; it's a +1. Most things that feel like they substantially increase a monster's challenge can be balanced as 'just a +1', and that's usually close enough.
Still, we don't want to throw off our damage calculations. We said we wanted this to be a two-part source of damage--once from the initial attack, and once when the Broodling claws its way out of you. Still, because the attack is only on one turn, it will count all as damage on one turn for balancing purposes. We're trying to get to 16 still.
But, this doesn't need to be CON-based. I think it makes sense if it is Dexterity-based for the attack; the spider's attack is quick. I also think this should be a larger variance than our spit. To get up to 16, with no multiattack, we can do 2d8+DEX for the first attack, and 1d10 for the second. To make this work, let's give our creature a +2 to DEX; that feels fine, it should be fast and nimble, but not super fast and nimble. It brings our damage up to 16.5.
So now our creature has +1 in defensive stats and -1 in offensive stats, just as we wanted, with an extra +1 from its ability to spawn things. By default, our spider probably has an AC of 10+DEX, unless it has some other influencing trait like a natural armor. We already set our DEX to +2, so that would make an AC of 12, which our chart tells us is a -1. That works out well since we have a bonus +1 from the spawn ability to cancel out. Yes, it means it has a somewhat low AC and is easy to hit, but I don't want to reduce its ability to hit because we need to make sure it actually gets to use the parasitic implantation ability.
For its to hit? D&D usually uses +DEX +Proficiency for ranged attacks, but that would only give us +4 – technically balanced, but a little low when we want to ensure that our creature actually gets to use its special ability. A lot of standard creatures get what is essentially a rogue's expertise feature to their attacks for balancing reasons, so let's go ahead and give our Broodmother that as well; adding the proficiency bonus twice. Now it is a +6, which feels a lot better as the source for a +0 in calculating the difficulty.
And there we have it! AC, HP, to hit, damage, a special ability, multiattack... everything nice and ready. I'll polish it all up and put it together for sale.
Converting to CR & Getting Ready for Publication
As part of readying this creature for sale, I want to convert it to normal D&D's Challenge Rating system rather than "Journeyman-tier and Pair-strength". The CR guidelines are buried in the Dungeon Master's Guide--a really good tool that is horrendously laid out, making it close to impossible to find relevant information. The chart is on page 274 for those following along at home.
So let's consult the chart! Our HP was 66, which translates to a defensive CR of 1/2. The AC for that is 13, which is not off by more than 2 from our AC of 12, which means we do not have to adjust it (if it is more than 2 away, you bump the CR up or down accordingly, per the DMG).
Our Damage Per Round was 16, which is an offensive CR of 2. That has a paired attack bonus of +3, but our attack was +6, so we'll bump the offensive CR up accordingly to a 3.
Now, we just average our offensive and defensive CR. 3 and 1/2 meet at 1.5 (I mean, mathematically, it's 1.75, but when we move along the rows of the chart, they meet between CR 1 and CR 2). This means we sort of just pick whether it is CR 1 or CR 2. I think, with the spawning ability, our creature is probably a little on the tougher side, so we'll call it a CR 2.
Final Product
To accompany our Broodmother, I went ahead and also created the Broodling that gets spawned by our creation. I also made a "Webspinner" that can ensnare the PCs and keep them from moving around.
Notice how this would pair well with our Broodmother tactically? The Webspinner can try to keep them from pursuing the Broodmother, but if the party focuses on taking out the Webspinners, they're leaving themselves open to the Broodmother spawning a bunch of Broodlings and swarming them.
This little collection of 3 spiders, all properly formatted with CR and everything, is available on DMsGuild as a pay-what-you-want title! Please consider supporting the site (and helping me pay for the site’s upkeep and hosting) by purchasing a copy!
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