Upgrading a monster in DnD 5e

Here’s a worked example of how I’ve used the Creating a Monster guidelines in the 5e DMG to  adjust a monster stat block to fit what I wanted for an encounter.

The base monster I’m using is the Ankheg (Monster Manual, page 21). It is rated Challenge 2. For my 5-character, 5th-level party I wanted a monster of Challenge 6 to finish off the small side-quest they were on (context in my last post). I decided to boost the Ankheg stat block to represent an ankheg queen. I used the Creating a Monster section of the Dungeon Master’s Guide (pages 273-279).

Guideline statistics

The DMG provides a nice simple table of the key combat stats by Challenge Rating, and I’ll just pick out two lines of it here.

CRProficiency BonusArmor ClassHit PointsAttack BonusDamage per RoundSave DC
2+21386-100+315-2013
6+315146-160+639-4415

Those are guidelines, and obviously not every monster precisely follows them. The base Ankheg has the following values, with the CR2 line repeated for comparison.

CreatureProfArmor ClassHit PointsAttack BonusDamage per RoundSave DC
CR2 Guideline+21386-100+315-2013
Ankheg+214/1139+51313

As you can see, the Ankheg has a slightly higher AC, partly offset by the lower value that applies if the creature is prone. It has much lower hit points: under half the guideline. Its attack bonus is high but damage slightly low. It looks as though the creature is overall under-powered, but it has a grapple effect on its bite preventing its target moving away and giving advantage on repeat bites against the same target, and an alternative area attack. It also has tremorsense, making it difficult to hide from. I infer that these special abilities balance the modest hit points.

So let me aim to adjust the stats for the ankheg queen to give a similar profile relative to the Challenge 6 guidelines.

CreatureProfArmor ClassHit PointsAttack BonusDamage per RoundSave DC
CR6 Guideline+315146-160+639-4415
Ankheg Queen+316/1180++830+~15

Filling out the stat block

With those guidelines in mind, I will construct the queen stat block in a little more detail.

Size, type and alignment

The base Ankheg is Large so I will take the Queen up to Huge. This gives her a 15ft space. There’s no reason to change the type from Monstrosity or the alignment (were I using it, which I’m not anyway) from Unaligned.

Armor Class

Increasing the natural armor to 16 seems reasonable. A larger monster would have a thicker exoskeleton and I don’t propose to change Dex. I won’t add to the lower AC of the vulnerable parts exposed when prone.

Hit points

The base ankheg gets 39 hp from 6 dice (d10s as a Large creature) and 6 times a Constitution bonus of +1. I want to double this (or more, because I’m not adding much except numerical stat increases and because I know this boss is going to be ganged up on at the end of a short day by 5 fifth-level characters with their shiny new Extra Attacks and level 3 spells and their freshly-increased Sneak Attacks and cantrips). I feel a Constitution bonus of +3 would not be amiss, and a Huge creature get d12s for hit points, or 6.5 on average. So that makes 9.5 hit points per die. Maybe 10 dice is right then, for hp: 95 (10d12+30)

Speed

The base Ankheg has ground speed 30ft and burrowing 10ft. I’ll bump that to 40ft for a larger creature, but not increase the burrowing.

Ability scores

Strength  I want the Attack bonus to increase from +5 to +8 while the proficiency bonus increases from +2 to +3, so I need the Strength bonus to increase from +3 to +5. Strength 20.

Dexterity  I don’t think this needs to change. Dexterity 10.

Constitution  I’ve decided while establishing hit points that the bonus should be +3. Constitution 16.

Intelligence  The base Ankheg has 1. As the leader this should be higher, but it’s still fundamentally a bug. Intelligence 2.

Wisdom  The base ankheg has 13 and again I’ll bump it up a little. Wisdom 15.

Charisma  The base ankheg has 6 and this should increase noticeably for a leader and scene climax boss. Charisma 10.

Saving throws, skills, languages

The base ankheg has no save or skill proficiencies and no language, and I don’t think this should change for the queen. Note that this makes it relatively vulnerable to spells and special abilities from higher-level PCs, and is part of why I wanted to be a little generous with Constitution, Wisdom and Hit Points.

Senses

The base ankheg has darkvision 60ft and tremorsense 60ft, which I think can be unchanged for the queen. Passive Perception improves with Wisdom modifier to 12.

Actions

I’m going to give the queen the same actions as the base ankheg, but uprate the numbers appropriately.

For the bite, we’ve established that the hit bonus becomes +8. I’m going to give reach 10ft rather than just 5ft, but still just one target. The total damage should at least double. For the base ankheg total damage is 4d6+3 (3d6 slashing, 1d6 acid, 3 Strength added to the slashing). So I’ll say 9d6+5 and split it 6d6+5 for the bite and 3d6 acid. I’m shifting the balance towards the acid because encountering the queen as a solo boss in the egg chamber is giving me an Aliens vibe rather than just giant ants*, and because the party’s tanks can reduce slashing damage. The escape DC for the grapple is 13 which I think is based on Strength modifier and proficiency bonus, so I increase that to 16. The base ankheg can grapple a creature up to its own size category, so I will maintain that, which for the queen is Huge.

* I rather hope my 11-year-old players haven’t seen Aliens, but I’m doing this for my own satisfaction too.

The base acid spray recharges on a 6, covers a 30ft by 5ft line and does 3d6 acid damage or half on a successful save (Dex DC 13). I’ll improve the recharge to 5-6, otherwise as a solo it is very unlikely to get a second shot and as I said I do want to emphasise the acid. The area can increase to 50ft by 5ft, and the damage can double in line with the bite: 6d6 acid damage. The save DC looks to be Strength-based (or maybe Constitution-based with an arbitrary +2?) and so (either way) it should increase by 3 to 16.

So the final stat block is:

ANKHEG QUEEN

Huge monstrosity, unaligned

Armor Class 16 (natural armor), 11 while prone

Hit points 95 (10d12+30)

Speed 40ft, burrow 10ft

StrDexConIntWisCha
20111721510
+50+3-4+20

Senses darkvision 60ft, tremorsense 60ft, passive Perception 12

Languages

Challenge 6 (2,300xp)

ACTIONS

Bite. Melee Weapon Attack: +8 to hit, reach 10ft, one target. Hit: 26 (6d6+5) slashing damage plus 10 (3d6) acid damage. If the target is a Huge or smaller creature, it is grappled (escape DC 16). Until this grapple ends, the queen can bite only the grappled creature and has advantage on attack rolls to do so.

Acid Spray (Recharge 5-6). The queen spits acid in a line that is 50 feet long and 5 feet wide, provided that it has no creature grappled. Each creature in the line must make a DC 16 Dexterity saving throw, taking 20 (6d6) acid damage on a failed save, or half as much damage on a successful one.

What do you think? Do you use this method of devising variant monsters? Does my ankheg queen feel like a Challenge 6 monster to you? Let us know in the comments.

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