Pages fly as your thumb flips through the Monster Manual. You are looking for just the right creature to throw at your players, something that fits the scene you have spent so long crafting. Nothing seems to be fitting as you near the end. What are you going to do? Your perfect scene has no monster in it, or at least one you don't have any stats for!
When I first started out GMing this was the kind of scenario that I ran into. I was new to the game and didn't really understand how crafting a monster went. With help of the Dungeon Masters Guide (DMG) and surfing the web I began to experiment more with changing up monsters that had already been started and eventually making some original creations.
Aside from the opening example there might be a few reasons that you modify a creature or monster. First comes down to what makes a monster or creature frightening. By modifying a creature that most players may already know you create an element of surprise and mystery. I know many players who have read through the MM either out of boredom or because they themselves are GMs. In this case you need to change things up to throw off those players. After all the MM is really just quick way for you to get some monsters on your table with less work. You should be able to play Dungeons and Dragons with just the player handbook.
Another reason you might modify a monster is to tailor it to be more of a challenge for your players. Perhaps your party is full of wizards, then a magically resistant creature might be more of a challenge but the theme of the dungeon is undead. You can take those preexisting creatures and make them more of a challenge than they might otherwise be. On the flip side you could also make a creature less of challenge. I have seen this done with Beholders on a few of the podcasts I listen to.
Finding a Foundation
Before you can modify a creature, you need to find which creature you plan to modify. I will take a recent example of a creature I worked on, a Crystal Guardian. For some context this is essentially a crystal golem that was tasked to protect the entrance to a long lost city. It is made of crystal because that is what the city was made out of.
If you look in the 5th edition MM you won't find a Crystal Golem, but you will find other golems. In my process I took one of the nastier golems, the Iron Golem, and made some modifications to it (which I will outline later). If possible I suggest trying to find a creature that matches a Challenge Rating (CR) that you are looking for (another topic outlined later) to reduce the amount of work you have to put into the creature. You want to look for a creature that shares similar abilities to what you want. If you are making a new type of dragon, say a Temporal Dragon, then you will want to pull from the Dragons. Even if your creature has nothing in common with any other creature you can still steal the stats from a creature as a template, you will just have a little more work to do.
Abilities and Skills
The first thing to start doing to your creature is to modify the skills and abilities that the creature has. The abilities are what make the creature unique. We know Doppelgangers because of their shape shifting, and Beholders because of their eye stalks and anti-magic cone. You can either create your own ability, which can be really fun, or you can take an ability from another creature. Don't worry about a balancing it at this stage since you are trying to shape the creature into your own.
This is not to be confused with Ability Scores. Later you will want to modify these scores if they need any tweaking, but for now simply note which ability scores are important for this creature. Does the creature have super strength? Then its strength will be important. Is it dumb as a box of rocks? Then its intelligence is going to be low. We can work on balancing these later when we get into CR.
Going back to my Crystal Guardians I stole an ability from another creature, one not even in the 5th edition MM. I found a CRD for D&D 3.5 for a Gem Dragon and stole its Ray Reflection ability. This ability would return any ray spell cast at it to its owner. Sadly this didn't get used as my players didn't have such skills, but I got to show an NPC injuring themselves with it.
This is probably the most important part to your monster. How do you present it to the party? How does it look? Maybe it has large spikes and tentacles, but your players won't know that until you describe the beast. Perhaps something about its description hints at its abilities. Go into detail about how the abilities look and feel. Describing the sensation of the hairs on the back of the characters necks just as the throat of the massive lizard begins to glow will give a much bigger sense of dread/excitement as the creature uses its lightning breath weapon.
This is a topic I've been trying to put off. There are a lot of issues when it comes to the CR system in Dungeons and Dragons but little to no fault of Wizards of the Coast. For those unfamiliar with the CR system, it boils down to giving each creature a Challenge Rating based on their abilities, damage output, health, and defensive features. That CR also has a number of experience points associated with it. In the DMG there is a fancy chart that helps you figure out how big of an experience pool an encounter based on difficulty and the number of players for certain levels. From here you can figure out just how many creatures you are able to throw at your players for an encounter for it to be of a certain difficulty. At least that is how its sold.
Going back to my Crystal Guardian, in theory it should have been considered a deadly encounter but just barely. I knew it was going to create a problem but from experience I learned that usually players can destroy one big creature in a deadly encounter because of action economy, an important subject that honestly deserves its own article. However I learned incredibly quickly that this creature was the deadliest monster they had faced. About 4/5ths of the party had little to no resources that could do anything to it, simply because it was immune to nonmagical attacks that weren't adamantine. On top of that it was immune to force damage, which halted one of the players. Luckily it was my intent to make this creature terrifying as there were about twenty more scattered throughout the city they arrived in.
This example shows one glaring flaw with the CR system. The CR system does not take into account the abilities and equipment of the party, the situation the players are in are crucial to this. With this in mind it is up to the GM to be mindful of the parties situation when using the CR system. When faced head on a party of five can easily wipe out a band of Kobolds, but when the Kobolds are dropping boiling oil from a murder-hole HALF the number of Kobolds could wipe a party. Keeping this in mind I use the CR system as a starting point, or a guideline rather than an end-all-be-all.
Keeping in mind the flaws of the CR system there is still some ways you can balance a creature when modifying it. For a small number of minor tweaks to a creature you don't have to worry about how much it off balance you make it. You should be mindful however when adding many minor tweak or very large ones.
Iron Golems in 5th edition have quite a few damage immunities. When I was crafting the Crystal Guardians one of the things I decided to tweak (regardless if crystals work this way) was they were also immune to force damage. In my opinion an immunity could be considered a fairly big change, after all you are canceling an entire type of damage. For this I decided to add a vulnerability to help balance that addition, vulnerability to thunder damaged. I saw this fitting because the spell shatter specifically gives creatures made out of crystal disadvantage.
Often I find myself having to increase or decrease the CR of a creature when working it as a guideline. I do this by adding abilities, defensive features, or health without removing or decreasing another skill. Additionally you can lower the CR of a creature by removing abilities, defensive features, health or attack damage to make the creature weaker. Lets get into detail about how changing each of these effects the creature overall, not in terms of its CR.
These are the bread and butter of your creatures. Changing abilities will result in vastly different outcomes depending on the ability. By adding another eye ray to a Beholder you decrease the chance of other eye rays happening but also introduce another potential problem for the players to deal with. Adding a breath weapon to a creature give them a way to hit multiple targets. Maybe the creature turns to a mist when dropped to 0 hit points. Abilities often are the most unpredictable when it comes to how they will effect a creatures balance. Often these change the flow of encounters.
This is a creatures' Armor Class, Damage Immunities, Resistances, Vulnerabilities, and Condition Immunities. The combination of these will determine how much damage the creature is actually taking. Immunities and Resistances will slow down the damage done in a fight and will make players question which one's they have until they use that type of damage or effect. A deadly game of twenty questions. Vulnerabilities will speed up combat once they are discovered.
While defensive features determine the rate of damage, health determines how much damage is need. In combination with defensive features this will determine the amount of time combat with the creature will take. Increase health, increase the amount of time the fight will last.
Simply put, this is on average how much damage the creature does in a given turn. A creature that attacks twice will on average deal about twice as much damage as a creature with only one attack, assuming they both use the same type of attack. This basically puts a time limit on the encounter. The more damage the creature does, the sooner the players have to kill it before it kills their characters.
One of the thoughts you should be having about your monster is how it thinks. Why is it attacking your players? Is it hunting them? Is it scared? This will really sell your monster more than any descriptions will. This makes them a part of your world, a living breathing monster. While the monster may not seem terrifying to you as a GM that is in part because you understand it, you know what there is to know about it. That isn't the case for your players, and when it seems to act in a way counter to what they might believe, that can be all the more terrifying.
Hopefully I was able to help you craft cooler monsters and creatures for your games. Let me know what cool creatures you have come up with either in the comments or on twitter! Now get out there and Frankenstein up some monsters!
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