Making 5E Monsters on the Fly, Fiction-First; the DMG in Your Head

This will let you improvise creature stats on the fly.

NOTE: These are not meant to approximate or reproduce any existing statblocks. Creatures done this way *I think* will be tougher than those of equivalent HD in the official stuff. They'll definitely have more hp and will likely have higher to-hit values. Based on my playing of 5e so far, that's a good thing. PCs getting too cocky these days.

I HAVE NO IDEA IF IT IS "CORRECT", but it seems fine from where I'm sitting. There is no warranty.

HD type: by size

• d4: tiny
• d6: small
• d8: medium
• d10: large (+1 damage dice, +2 damage)
• d12: huge (+2 damage dice, +4 damage)
• d20: gargantuan (+4 damage dice, +8 damage)

Example: a hobgoblin may do 1d8 + 4 damage with its Morningstar. If it were made huge (giant size), it would gain +2d to damage and +4 flat on top, for 3d8+8. 

HD value: don't roll; just max it

# of HD: assign tier first (Basic / Expert / Champion / Master / Immortal), then number of HD within that range by combat role or by scariness:

• caster: least HD / scary 
• mixed caster: next least HD / scary
• martial: next most HD / scary
• tank: most HD / scary

Assign proficiency and gear by what seems legit; don't worry about what's permitted; worry about what seems true.

Instead of additional attacks, casters have spell slots.

A caster can use all attacks for spell slots and still have 1 physical attack left, but it's offset by not being proficient with good weapons and armors.

Assign spells to casters by what seems legit. 

Stats by HD range

• HD 1-4 (Basic); +4 d+2 x 1; ST 2/0/-2; DC 12
• HD 5-8 (Expert); +6 d+3 x 2; ST 3/1/-1; DC 14
• HD 9-12 (Champion); +8 d+4 x 3; ST 4/2/0; DC 16
• HD 13-16 (Master); +10 d+5 x 4; ST 5/3/1; DC 18
• HD 17-20 (Immortal); +12 d+6 x 5; ST 6/4/2; DC 20

How to read those statblocks:

• HD X-Y: how many hid dice the creature has, corresponding to its tier
• +X (attack) d+Y (damage bonus) x Z (number of attacks or spell slots or combination thereof)
• ST X/Y/Z (saving throw bonuses: good/okay/bad
• DC X: difficulty class of saving throws 

Example

A cursed knight, very scary. 

I use the expert tier cause she's serious business but still possible for a starting party to take on if lucky. Since she's very scary, I use the most HD possible from the expert tier (HD 8). So that makes her:

• AC 20 (plate plus shield)
• HD 8d8 (64 hp)
• Attack +6 for 1d8+3 damage, two attacks
• Saves: Strength and Dex: +3, Wis -1; else +1
• Save DC 14 (say for martial effects, like being tripped)

A Very Large Snake of Human Bones, Pelvises for Jaws

(How to run this: give a saving throw when a PC would suffer harm. Describe what is about to happen to them and ask how they try to avoid it. Have them make the throw. On a success, they avoid it as they said. On a fail, they do suffer the harm: describe it. The monster's "when clauses" interrupt things like attack rolls. If a when clause triggers and it has "or-separated clauses", roll 2d6: 6-, first clause; 7-9: middle clause; 10+, final clause.)

I gather bones into my body, for in time I must molt. All surfaces are floors to me. 

When they approach me with violence, I strike, or I move through and past them to tear their flesh and weaken them, or I break apart and let my bones rattle still.

When I strike, I swallow one of them, or spit my blinding poison, or snap off an arm or leg in my jaws. 

When I break apart, I reform upon anyone who lingers in my bones, and my ribs will pierce them and rip them apart. 

When they try to hide, I discover them suddenly, or make my way noisily to them, or perhaps I am bored and leave. 

When they try to run, I strike, or pursue, or perhaps I grow bored and let them. 

When they hope I won't notice, I do and I strike, or I do but I don't let on, or I do and don't care. 

When they invoke The Lord of Serpents, let them prove their devotion the holy one, and I will lie on my back, open my ribs, and convey them into whatever darkness they wish. 

Edition-Math Conversion Compendium

Perhaps this is my final word on the subject. I'll try to make this post exhaustive, with a lot of variants, and will update it if necessary in the future.

The Situation

You've got some older edition monsters. They don't have a level; they have a certain number if HD. You need to convert these monsters to 4E.

You can do this modularly.

For instance, in my current campaign, I'm using:

• PC Variables
• HIT POINTS NIGHTMARE MODE (no hp increases by level)
• General Math Bloat Fix (no 1/2 level bonuses across the board)
• Monster Variables
• HD to hp: 1/2 HD hp
• Level 0 hp: 3 * HD
• Threat Jump: Simple
• Damage: Hardcore Mode (thought I'm tempted to do NIGHTMARE MODE, always bold and all caps)
• Progression: Staggered
• Powers: mostly sticking with the vanilla role features and adding in miscellaneous stuff as I see fit; examples to come in later posts.

While this is written with converting from HD to level in mind, you can also use the info here to spice extant 4E critters to taste, taking off some level 0 hp, staggering their stat progression, whatever.

Variables

You can adjust several things on the player side and on the monster side. 

Note: "RAW" = "Rules as Written" = what the 4E rules actually say.

PC Variables

HP Advancement

• You now have a class HD that may or may not be used by GM
• It is: d[(class hp/level * 2) - 2]
• Example: Wizards have a d6 HD
• they gain 4 hp/level RAW
• (4 * 2) - 2 = 6 = # of faces on class HD
• Example: Paladins have a d12 HD
• they gain 7 hp/level RAW
• (7 * 2) - 2 = 12 = # of faces on class HD
• Standard: do not mess with hp advancement. 
• Hardcore Toggles:
• 1/2 RAW hp gained per level
• Roll class HD at first level:
• Gain that many hp + Con mod
• Gain that many hp + Con score
• 1/2 starting class hp 
• step down each class's HD by one size
• use Con bonus rather than Con score when figuring starting hp
• NIGHTMARE MODE: do not gain hp/level

Half-level Bonus

• General Math Bloat Fix: do away with it for all player statistics, including skill bonuses, initiative, attacks, and defenses, and reduce DC of every listed DC exceeding 19 by (DC - 19) / 2. 
• Example: DC 18: untouched since <= 19.
• Example: DC 20: (20 - 19) = 1. 1 / 2 = .5. Round down. No change. 
• Example: DC 27: (27 - 19) = 8. 8 / 2 = 4. 27 - 4 = 23. 23 is final adjusted DC.
• Hardcore Mode: do not reduce RAW DCs for rituals, etc. 
• NIGHTMARE MODE: do not do 1/2 level reductions for monsters. 

Monster Variables

Note that, in 4E, monsters have 20% higher chance to hit relative to B/X. A level 1 monster needs only a 10+ to hit AC 16 in 4E; in B/X, that required a 14+. So, even though average damage expressions may be lower in 4E, the damage over time tends to even out since the monsters will hit more often. Keep this in mind when choosing your level 0 damage below.

• HD to Level Conversions
• Simple: 1 HD = 1 Level
• Threat Jump:
• 1 HD = Level 1
• 2 HD = Level 3
• 3 HD and up = Level [HD * 2]
• For an hp progression proportional exactly with that of B/X:
• Level by HD as above
• Subtract from the standard hp for that level the following:
• -11 (artillery/lurker)
• -13 (soldier, controller, skirmisher)
• -15 (brute)
• This will double level 1 hp at level 2, triple it at 3, quintuple at 5, etc.
• HD by Role (these are not options; these establish terms)
• 1/2 HD: d4 HD
• HD - 1: d6 HD (artillery, lurker)
• HD: d8 HD (skirmisher, controller, soldier)
• HD + 1: d10 HD (brute)
• HD + 2: d12 HD  
• HP per HD
• full: d8 HD = 8 hp/level (or, if rolled, 2d8)
• half: d8 HD = 4 hp/level (or, if rolled, 1d8)
• Level 0 HP
• HD * 1
• HD * 2
• HD * 3 (Standard)
• Level 0 Stats (these do not change; I'm supplying them as a baseline)
• Attacks: +5 vs AC; +3 vs NADs (Non-AC Defenses)
• Defenses: AC 15; NADs 13
• Skill DCs (to escape/overcome): DC 11
• Level 0 Damage (min < avg < max)
• Standard: 1d8 + 4 (5 < 8.5 < 12)
• Hardcore: 2d8 + 4 (6 < 13 < 20)
• NIGHTMARE: 2d8 + 8 (10 < 17 < 24)
• INSANITY:
• 1d20 + 10
• 3d12
• Monster Progression
• Standard: 
• +HD in hp / level
• +1 stats / level
• Staggered
• HD / level as above
• odd levels: +1 attacks and skill DCs
• even levels: +1 defenses and damage (try to arrange the dice and static portions of damage such that the total of die faces is double the static damage; increase the static damage first, then the size of the dice: e.g., 1d8 + 4 > 1d8 + 5 > 1d10 + 5 > 1d10 + 6) 
• Weird: advance monsters via some combination of ad-hoc traits and stat bumps (or not!) that seem appropriate
• Morale
• Base morale: 7
• If the monsters fail a morale check, they flee/surrender/offer terms
• Failing a morale check means rolling over morale on 2d6
• Make a morale check:
• When the monsters suffer their first casualty
• When half the monsters have been subdued
• When the leader (if any) has been subdued
• In the case of a single individual, when it suffers its first hit and again when first bloodied
• If an individual or side passes two morale checks, it fights to the end
• Monsters may also flee is that is obviously what they would do; this system is intended for when the GM is not sure what would happen
• Monster Roles (choose 1 and, if appropriate, add leader role as well)
• Skirmisher
• d8 HD
• As part of standard attack action, can shift up to half speed before/after attack
• Artillery
• d6 HD
• +2 to attack
• Brute
• d10 HD
• -2 AC
• + 25% damage
• Controller
• d8 HD
• On miss, apply weak status effect (prone, slow, push/pull 1 multiple or 2 single); on hit, stronger effect (immobilized, dazed, restrained, 5 ongoing damage, push/pull 2 multiple or 3 single, slide 1 multiple or 2 single). 
• Leader
• +2 morale if chief; +1 otherwise
• allies who can sense the leader's presence gain +1 attack or +2 damage
• Soldier
• d8 HD
• +2 AC
• enemies in aura 1 provoke opportunity attack on shift or on attack that doesn't include this soldier
• Lurker
• d6 HD
• +1d8 damage and 1/2 damage on miss when has CA or when target is isolated
• Monster Powers
• Not necessary
• But you can take one from an existing monster if you like
• You can always reduce a feature to gain a power: e.g., reduce defenses by 2 to gain +ongoing damage on a hit.
• Just whatever seems legit
• In general, I wouldn't mess with it—just port over the "powers" of the older edition monsters
• That said, I'll post a few of my monster alterations in the future

Grounded 4e

NOTE: all the math here has been updated and expanded with a ton more options; see here: http://detectmagic.blogspot.com/2014/03/edition-math-conversion-compendium.html.

There are a number of pretty simple, mostly GM-facing hacks you can do to make 4e play more grounded.

No Math Bloat

These changes make leveling more about new abilities than new numbers. It makes gear much more important. It makes monsters really scary. It probably means you want to use something other than monsters for XP bags (like quest award XP or XP for treasure). 

• Gaining a new level does not by itself grant additional hp
• Remove half level bonuses across the board
• Scaling published skill DCs higher than 19:
• For Rituals with ranges of effect:
• Either leave them alone, to let some ranges be unobtainable at low level, or:
• subtract 19 from the DC
• divide remaining DC by 2, rounding down
• Add to 19 for final DC
• Example (Ritual: Chorus of Truth)
• 19 is the high bound of the lowest range of results. We leave that be. 
• The next range includes 10 results. We reduce that to 5: 20-24.
• The next range is 10 as well. That gets reduced to 5: 25-29.
• The final range is just one number higher and up: 30+. 
• For DCs that are just high because the PCs are presumed to also be of high level: 
• ignore the DC
• just use 11/12/19 range as given in the DM cheat sheet below
• For monsters:
• After determining their level (if converting from HD-determined monsters), either:
• Leave the levels alone, or:
• Remove half their level from their attacks, defenses, initiative, and skill checks

I think I like to just leave them alone and let higher level monsters be really hard to hit.

Converting pre-4e Monsters

To convert an older monster to 4e style:

• damage
• give it damage expressions like so: http://slyflourish.com/master_dm_sheet.pdf
• 1d8+3 + 1/level (1d8+4 at level 1)
• -2 for area
• +2 for limited
• +2 for brute
• +4 for brute limited
• minimum damage for minions
• on top of normal damage, add flat (nonscaling) +1d12 
• this brings the average damage expression to an average of 15
• retain any special abilities (but not multiple attacks probably unless elite or solo)
• determine how much hp it should have:
•  by hand:
• < 1 HD: minion
• HD - X: 7 + 6/level (13 at level 1)
• HD + 0: 9 + 8/level (17 at level 1)
• HD + 1:  13 + 10/level (23 at level 1)
• HD + 2: elite (double a +0 monster's hp, 34 at level 1)
• by the DM sheet above:
• <1 HD: minion
• HD - X: low - 11
• HD + 0: standard - 13
• HD + 1: high - 15
• HD + 2: (low * 2) - 11
• to determine what 4e level a monster of a given HD should be
• HD1 = Level 1
• HD2 = Level 3 (hp = level 1's hp x 2)
• >= HD3 = Level [HD*2]

This preserves the double-hp jump from HD1 to HD 2 and the steadier HP increase thereafter. The increased damage across the board likewise makes the first level much more difficult for PCs. The hp subtraction from monsters makes sure they can go down in a hit or two, as in days of yore. 

Other Tweaks

• All creatures (including your allies) provide cover, and may be struck on miss by 5+. 
• Track encumbrance for real (more in an upcoming post)
• Whenever they take an extended rest, do something bad. 
• Make dungeons dark, apply RAW dim light penalties
• Consider capping advancement at 10 or 11.
• Restock dungeons
• Wandering monsters (10% chance each 10 minutes)