I have been playing D&D with my friends for not too long and they said they would like me to be the next DM. I think it would be fun, but I'm not sure how customizable I can make the enemies in my game. For anyone who is usually DM, do you have advice? Should I follow the monster manual closely or should I treat it more like templates? Any help would be greatly appretiated.
The books leave it up to you.
DMG 273-279 contains information for building monsters. The section opens with:
The Monster Manual contains hundreds of ready-to-play monsters, but it doesn't include every monster that you can imagine. Part of the D&D experience is the simple joy of creating new monsters and customizing existing ones, if for no other reason than to surprise and delight your players with something they've never faced before.
If you really want a very detailed tutorial on building your own custom monsters, you can check out AngryGM's series on building your own monsters, which I found super helpful.
Beware! Building your own monster is often a trap. As you can probably surmise from AngryGM's articles, it takes a lot of fiddling with numbers and real or simulated playtesting to get a good monster. And after all that thinking, your party will take it down before it uses half of its super cool abilities (ask me how I know...).
I recommend using the MM directly to get experience
While you can start with custom monsters, I think it will really help to hew to the pre-built monsters, and use those CR guidelines. Not only will you get a feel for how the mechanics of the game works, but you will also eventually get a feel for how your players and their characters can take certain types of encounters, as well as your own GMing style. All of these factors can influence what kinds of monsters you can throw against your players, and give you ideas for custom monsters.
What you're suggesting is homebrew. The monsters in the MM are built so that your party can deal with them appropriately, and altering them can make them over or under powered. Without proper planning, testing, and balancing, you could create anything from something that is practically un-damageable, but deals practically no damage, to a glass cannon, or even something that is far and beyond anything that your party can even last a few rounds with.
In particular, the main thing you want to focus on is each monster's CR (Challenge Rating). This indicates the difficulty of a monster, in relation to the party. You can read more about Challenge Rating here
For a first time game, I would strongly recommend sticking to the Manual. The only thing I might change is the appearance, to fit a theme, instead of altering the statistics or abilities in any way. For example, instead of "Cultists", a group of "Possessed Druids" attack the party.
Once you get used to the way that creatures are balanced, then you might be able to explore alterations.
If you can, sure!
Knock yourself out, really. Certainly, customizing monsters can be fun for your group, as they enjoy the creatures you made; and you, as a DM, as you engage in the game design aspect of the hobby.
As someone who likes to customize creatures a lot, your only enemy here is exhaustion.
Homebrewing or even just adding bits to existing monsters takes a lot of work in number crunching (looking up and down the DMG and back to the calculator), careful thought in the design aspect (thinking up what would be fun to add/modify?), and time in playtesting and tweaking (even after you think you got it just right); and it can be draining. Be careful that you don't burn yourself out in doing the additional work on top of the constant pressure to be done before the next session, and the actual campaign/story/dungeon to design.
*As much as you'd like...
Before you are several paths. Choose one:
Downhill path of least resistance Continue using the MM monsters as written. Just plug and play. This is the intended method, and works well with little effort on your part. Or maybe change up the tactics a bit, but not the rules of the beasts. Maybe they fight in coordinated 2s and 3s instead of alone (if smart enough) for example. And don't be afraid to have the monsters run away if they're not cornered and are losing; anything with half a brain wants to live.
Level path of only a little resistance Take what's in the Monster Manual and file off the edges. Make minor tweaks that maybe change the feel of monsters, but not the crunchy mechanical numbers and math parts. So maybe take the stats for a monster that fights the way you like, but completely change the description and name. Or maybe change an attack type around a bit, so it does the same damage but in a different way. This is my preferred path when GMing for highly experienced players who've memorized the rule books.
Gentle uphill path of some resistance Take a monster from the MM, but make it harder. (or easier.) Maybe increase it by a hit die or two. Or give it a slightly better attack or AC. Or if it is intelligent and capable, give it a level or two of a PC class. Doing this gives you a baseline to work from, but then improves it. Remember to increase the CR and XP awards accordingly. This isn't as hard as starting from scratch and lets you get into the mechanics a bit, without going too deep into the weeds. I also do this with experienced players who've memorized the rulebooks. Because when they think they know all the metagaming tricks to defeating thing X, but thing X has a level of wizard or fighter, then thing X suddenly has an ace up its sleeve!
Uphill slope of tired muscles Take a book from either an earlier edition of D&D, or another RPG engine entirely. Find a monster in it that you like that isn't in your MM. re-write it using D&D 5e rules. This is challenging. The further from your rule base you go, the harder the adaptation becomes. But again, there's a framework to start from. And usually other monsters in the same book that ARE in your edition, so that helps you kind of figure out relative difficulty ratings, etc. And remember that if you're going this way, a perfect adaptation isn't as good as a workable adaptation. This will probably require some on-the-fly adjustments when you play it the first time or two, so be ready to tweak things to fit your party. Then adjust what you wrote and try it again later. (Playtest.)
Climbing the mountain This is taking something from outside an RPG. Be it a book, a movie, a myth, or whatever your source material is, do some research. Ignore rules while you gather all the descriptions you can of what it can or cannot do, how it fights, how it lives, etc. Then try to find examples of each bullet-point item in other monsters in your MM and borrow those. Then go back and invent things to flesh out the rest. This is hard and takes time. And your first draft, like a college research paper, won't be a polished, perfect, version of what you want. Be ready to revise and retry (playtest.)
Climbing Mount Everest This is blank-slate writing. Your imagination is all you have to go by. Slap down what you want. Then write down how you want to do that with rules. Again, be willing to adapt from existing MM entries. But this going to require you to study the rules, study other people's how to guides. Adapt, revise, and retry until you get what you want.
Don't be afraid to experiment. Be patient. Ask your players to be patient. And be willing to reward their efforts. If you think the monster is too hard for your players ("Oh crap, they're about to die! That wasn't supposed to happen!") don't be afraid to pull the plug on the encounter. You can either hard-stop with an honest apology, "Guys, I'm sorry. This wasn't meant to be so difficult. I obviously misjudged this critter. Stopping here, because you aren't supposed to be wiped out!" or you can soft-stop with escape routes or GM-level fudges. Maybe it suddenly goes from 250 remaining HP to 2. Or you encourage the PCs to escape down a side passage and don't have the creature pursue. Something to give the players a safe out, and let them know they should take it because of your errors, not theirs. If they are clear on why the encounter is failing -- that it isn't their fault -- they should be okay with that.
And if it goes well, then AWESOME! you've created something!
Personally, I'd start from the top, and work your way down. Each one of the above "paths" is progressively harder, but builds on the experiences you gained from the previous paths. So you can do a few examples from each, and as you master that way to approach it, you'll be better able / more comfortable trying the next approach. Your skills will improve as you "level up" to the next path.
There are three tiers of customization, in order of difficulty
- Cosmetic changes. Find a monster with an appropriate challenge level, and distort it beyond recognition. For example,
Brutehounds are feral, barely intelligent humanoids. They stand just under four feet tall, and their bodies are covered in black and white fur. They have elongated faces, with large unblinking eyes and visible teeth. Though they are capable of walking upright, they typically prefer to move on all fours. They communicate with harsh barks and yaps, although some brutehounds can speak common or gnomish.
Brutehounds have the same stats as goblins.
You may make some insignificant mechanical changes here as well. Perhaps our little badgermen have a very keen sense of smell. Instead of using scimitars and javelins, they bite and hurl stones. Be careful here, though, because what is and isn't significant depends on the party. If you have a player who loves to disarm their enemies, no scimitar is no fun. Being tracked through a forest by scent could be a cool scenario, but if your rogue relies on catching enemies by surprise, then being smelled coming 50 feet away is a non-starter. If someone's backstory is about how goblins killed their parents and they have devoted themselves to revenge, don't turn the goblins into badgers.
- Behavioral changes. Don't just describe them differently, make the monster act differently. Goblins are greedy, scheming, cruel little things who delight in pointless evil. They betray their friends at the drop of a hat. In contrast,
Brutehounds live in large, familial colonies in a communal burrow. They largely keep to themselves unless their home is threatened. Different brutehounds take on different roles in their colony: some work to protect the home, others forage, others raise children. A small number, typically the most intelligent among them, are responsible for trading and bartering with more intelligent races. Strangely enough, the colony does not have any kind of leader. Brutehounds organise themselves completely instinctively.
If their home is threatened, brutehounds defend it furiously. They have been known to go on mindless rampages if they are unable to return to their home (or if it is destroyed). They prefer to live in wooded areas, and the larger colonies have been known to domesticate bears for protection.
In combat, brutehounds will typically run away if they feel outmatched. Otherwise, they attempt to surround and overwhelm their enemy. If on a rampage, they simply attack whatever they can, giving little thought to strategy or their own safety.
Behavioral changes rarely require mechanical changes to back them up, but you should be careful not to make the changes lead to challenge issues. "If you hurt a brutehound, their entire colony will stop at nothing to hunt you down" is a problem, as is "brutehounds fight by throwing rocks at their enemies from tall cliffs" - both of these turn goblins into an encounter that's not goblin-level
- Significant mechanical changes or creating a new monster. I wouldn't attempt these a) without a good reason and b) without playtesting.
The main reason to do this would be that you have a vision for a particular encounter and no monster that seems appropriate. Maybe you want a monster that runs, swims, flies and burrows that the players have to chase for miles and miles. There's not much advice I can give here except to be prepared to make changes on the fly. If you've made it too hard, make it easier. Maybe it turns out it can only fly in short bursts, or it slows down in the sunlight. Don't tell your players that you're making changes, just act like it's all in the plan. Your goal is not to protect the integrity of the monster, it is to protect the integrity of the encounter.