Homebrew University: Making a Monster [5E BBEGs]

Monsters R Us

Join Demiplane Community Member Kyle Taylor (Stormchaser) in Homebrew University: a recurring blog that explores practical frameworks and perspectives to evolve popular Tabletop Roleplaying games to best fit you and your table’s tastes!

Hello Game Masters!

Over the last three episodes of Homebrew University, we’ve concerned ourselves with armour, weapons and magic items. A cornucopia of options for empowering your players and making them feel like heroes.

Now it’s time to make them feel fear.

The World’s Greatest Tabletop Fight Club

The writers of Dungeons and Dragons posit that the game is built around three ‘pillars’ of adventuring; exploration, social interaction and combat. Of these, the combat pillar is by far the most extensively supported by the game’s mechanics. The vast majority of the rules in the Player’s Handbook revolve around combat, how to deal damage and how to protect yourself from losing hit points.

Since DnD is such a combat-focussed ruleset, it’s only natural that monsters and NPCs, the creatures against which you fight, form an integral part of those rules. 5th edition even goes as far as to make those two categories one and the same – an NPC is just another kind of monster. So, what makes a good monster? From the perspective of a DM seeking to provide a stimulating and enjoyable gameplay experience to their players, the following things are important to consider:

A monster should be interesting. Players have seen bags of hit points that hit and get hit in return a thousand times before. Give monsters unexpected and unusual abilities that surprise your players and make them deepen their tactics beyond simply rolling to hit.

A monster should be impressive. Combat that feels too easy quickly gets boring. Design monsters that will catch your players off-guard and inspire respect for their opponents.

A monster should be balanced. A monster with characteristics that fall too far outside its intended CR range will make encounter difficulty a challenge to judge, potentially resulting in a fight that is either much easier or harder than it should be.

Perfectly Balanced, As All Things Should Be

The Dungeon Master’s Guide for 5e has a table on page 274, giving target values for the five most critical combat relevant statistics of a monster. These are armour class, hit points, attack bonus, damage per round and saving throw difficulty class. These assumptions underpin the balance inherent to the CR system and it’s important to bear them in mind when designing monsters of your own.

Given the known target CR of your creation, you can see your target values in each of these categories and reverse engineer to find your monster’s base stats. Below we continue to homebrew an example CR 2 creature:

Armor Class (AC): AC consists of a combination of dexterity and armour. Typically, a creature with an armour contribution of 3 to 5 should have no more than a +2 dexterity bonus to AC. A creature an armour contribution of 6 or more should have no dexterity bonus to AC. Supernatural creatures may operate above and beyond this convention in some special cases.

For example, a monster with a target AC of 13 might have a +1 dex bonus and 2 natural armour.

Hit Points (HP): Hit points are broken down into a number of hit dice (the size of each of which is defined by the monster’s size) with a constitution bonus added to each one. For each hit die take the average value. The creature’s number of hit dice also determines the maximum spell level available to spellcasting monsters in the same way as level does for full caster player characters.

For example, a medium monster with a target hp of 86-100 might have 12d8 hit die and a +3 constitution bonus, for a total of 96 hp.


Main Attribute: Since CR defines proficiency bonus in the same way as level does for PCs, this can be subtracted from the target attack bonus to determine the contribution from the monster’s main offensive stat.

For example, a CR2 monster with a target attack bonus of +3 has a +1 strength bonus.


Damage: Average damage per round can be broken down in several ways. It can be split into multiple attacks and each attack can be split up into average dice results and the appropriate attribute bonus.

For example, a monster with an average 15-20 damage per round can be represented as a bite attack that deals 1d8+1 damage and two claw attacks that each deal 2d4+1 damage, for an average of 18 damage per round in total.

Saving Throws: Much like the attack bonus, the attribute governing a creature’s save DC can be calculated by subtracting 8 plus its proficiency bonus from the target value.

For example, a CR2 monster with charisma based abilities and a save DC of 13 has a +3 charisma stat.

Thus, our example monster could have stats of 13, 12, 17, 4, 14, 16. A tough magical beast of monkey-like intelligence with mysterious supernatural powers! I had a roommate like that once…

Phenomenal Cosmic Power

I mentioned earlier that a monster’s maximum spell level is tied to its number of hit dice. The strange creature that we have produced in the above example might be balanced, but it isn’t yet especially interesting or impressive. It’s just a ferocious creature with claws and teeth like any other. To combat this we add other abilities and the easiest way to do this is to base them on existing spells. This works well, because spells already have their power level quantified by spell level, so it’s easier to judge whether an ability is of the right strength.

Our CR 2 monster has 12 hit dice, which is equivalent to a maximum of 6th level spells. A classic example of a spell of this level would be Flesh to Stone. Perhaps our monster has a petrifying gaze or bite that slowly transforms its enemies into solid rock? The flavour is up to you, but the mechanics can be quickly and easily derived from the basic rules – a DC 13 constitution saving throw, leading first to the restrained and then to the petrified condition. The possibility of being turned to stone is certainly an ability liable to ingender your players with a healthy respect for what such a monster could do to them!

This simple formula can be applied to almost any spell. Disguising these existing mechanics as inherent abilities is one of the quickest and easiest ways to add interesting and unusual abilities to the monsters that you create. If you wish to limit the number of times that a creature can use such an ability, then you can add a number of charges similar to a PCs spell slots, which provide a convenient guideline for this. An alternative design that requires less bookkeeping emulates the same mechanic used for a dragon’s breath weapon, by simply rolling a dice to determine if the monster can use the ability again.

Add unique flavor to your creature’s weapon attacks using our guide to creating unique 5e Weapons!

Achilles’ Heel

Next, consider adding a damage resistance and vulnerability pairing, in order to encourage your players to adapt their tactics to defeat the monster. Vulnerability to an unusual damage type often rewards intelligent play and deeper tactics, driving your players to use their brains to overcome a creature’s resistances. The most common example of this in the Monster Manual is resistance to non-magical bludgeoning, piercing and slashing damage, but this is easily overcome and only significant at lower levels. To challenge a higher-level party, consider adding absolute resistances that can only be overcome by using a different damage type.

For example, the vanilla skeleton is vulnerable to bludgeoning damage. If you want to make such a creature more threatening, then you might add resistance to both slashing and piercing to it. Your martial players will need to switch from swords and arrows to hammers and clubs in order to effectively deal damage. Similarly, undead are immune to poison and might well also be resistant to necrotic damage. However, a vulnerability to radiant damage makes sense for undead creatures. This weakness can give you room to raise the CR of the monster slightly, makes the fight more interesting and gives divine characters like clerics and paladins an opportunity to shine.

Don’t be afraid to add additional requirements for an easily accessible damage type to qualify for vulnerability! A Monster Manual example of this would be the Rakshasa, which is only vulnerable to piercing damage from magic weapons wielded by a good-aligned creature. However, this kind of special condition is also commonly seen in literature and old fairy tales. It was said that no man could slay the Witch King of Angmar and only the sword of truth could harm the wicked fairy Maleficent. If you players discover that they must fulfill special conditions to defeat a foe then that’s no longer just an encounter… that’s a quest!

Natural Talent

Some monsters, like goblins and minotaurs, share abilities with a hero class. In the aforementioned examples, these are cunning action and reckless attack, respectively. From a design perspective, this is done instead of giving the creature the full abilities of a heroic class, because this would make the monster unnecessarily complicated. Nevertheless, leveraging one or two existing abilities from the player classes is a great way to spice up what a monster can do and provide some more interesting martial options. 

Here’s a short list of common player abilities well-suited for use in monster building:

  • Action Surge 
  • Bardic Inspiration 
  • Battlemaster Maneuvers 
  • Brutal Critical
  • Cunning Action
  • Favoured Enemy
  • Feral Instinct
  • Fighting Style
  • Indomitable
  • Reckless Attack
  • Rage
  • Second Wind
  • Sneak Attack
  • Uncanny Dodge

 Usually, a monster needs no more than two such abilities to make it feel unique and different to more run-of-the-mill foes. On top of this, don’t be afraid to reflavour these mechanics to suit your monster better. As long as you have a firm grasp of what the mechanics represent, the world is your oyster! 

When Monsters Become Legends

5th edition introduced a new and powerful mechanic for building boss monsters, by making them ‘legendary’. Legendary monsters can be of any CR, but are especially powerful, because they gain additional ‘legendary’ actions that can be taken at the end of another creature’s turn. Each of the legendary monsters in the Monster Manual has three legendary actions per round and the reason for this is that Dungeons and Dragons is balanced around a party of four player characters. Thus, the monster acts on its turn and between every player turn, until its next turn. This forces the players to rethink their tactics, because they no longer have the safety of knowing that they have a full round before the monster acts again. In order to maintain this dynamic, a legendary monster designed for a party of a different size should have a number of legendary actions equal to one less than the party size.

Legendary actions come in many shapes and sizes, but a good rule of thumb is to include one melee option, one ranged option and one movement option. Sometimes a legendary action can combine two of these in one, at the cost of multiple uses. A good example of this is a dragon’s wing attack, which deals damage to all adjacent foes and launches the dragon into the air. Additionally, most legendary monsters will have a number of uses of legendary resistance, which allow the creature to succeed on a saving throw that it would otherwise fail. This legendary template can even be applied to existing monsters, if you want one to be particularly special or simply a memorable boss. Even if the base monster is something as small as a kobold chieftain.

Finally, give your legendary monster a lair. Lair actions are environmental effects that only occur when the creature is fighting on home turf. During a combat within the monster’s lair, a legendary creature can take one lair action per round on initiative 20 (losing ties). Typically, a monster will have a selection of three lair actions to choose from and cannot use the same effect on two consecutive rounds. Look to existing monsters in the Monster Manual for examples of the type of action that works well in a lair. Chromatic dragons are especially good examples, each with their own set of elementally themed features that can modify the terrain, impart conditions, or simply deal damage.

Seeking Legendary Inspiration? Grab your cloak and Rod of Warming and explore The White Dragon of Hulgald! Check out the legendary Planar Tale here. 

So, if you find your players gathering around the table for a special adventure over the holidays, consider throwing something new and exciting at them. An original monster that they’ve never seen before, because nothing is quite as scary as the unknown.

Have fun and I’ll see you next time on Homebrew University!


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