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