When running old AD&D1e modules for D&D 5e adventures, follow these guidelines…
FREQUENCY: Chance of encountering (1d100).
* 1 to 65 … common creature
* 66 to 85 … uncommon creature
* 86 to 96 … rare creature
* 97 to 100 … very rare creature
NO. APPEARING: Use as is.
ARMOR CLASS
* AD&D AC 10 … 5e AC 10
* AD&D AC 9 … 5e AC 11
* AD&D AC 8 … 5e AC 12
* AD&D AC 7 … 5e AC 13
* AD&D AC 6 … 5e AC 14
* AD&D AC 5 … 5e AC 15
* AD&D AC 4 … 5e AC 16
* AD&D AC 3 … 5e AC 17
* AD&D AC 2 … 5e AC 18
* AD&D AC 1 … 5e AC 19
* AD&D AC 0 … 5e AC 20
* AD&D AC -1 … 5e AC 21
* AD&D AC -2 … 5e AC 22
* AD&D AC -3 … 5e AC 23
* AD&D AC -4 … 5e AC 24
* AD&D AC -5 … 5e AC 25
MOVE SPEED
* AD&D Move 1″ … 5 feet
* AD&D 6″ … 20 feet
* 9″ … 25 feet
* 12″ … 30 feet
* 15″ … 35 feet
* 18″ … 40 feet
* 21″ … 45 feet
* 24″ … 50 feet
* 36″ … 80 feet
* 48″ … 90 feet
AD&D movement shorthand…
* #” … land speed
* /#” … flying speed
* //#” … swimming speed
* (#”) … burrowing speed
* ‘#” … web crawling speed
HIT DICE
* HD 1 to 2 … +2 attack, DC 9 save
* HD 3 to 4 … +3 attack, DC 10 save
* HD 5 to 6 … +4 attack, DC 11 save
* HD 7 to 8 … +5 attack, DC 12 save
* HD 9 to 10 … +6 attack, DC 13 save
* HD 11 to 12 … +7 attack, DC 14 save
* HD 13 to 14 … +8 attack, DC 15 save
* HD 15 to 16 … +9 attack, DC 16 save
* HD 17 to 18 … +10 attack, DC 17 save
* HD 19 to 20 … +11 attack, DC 18 save
HIT POINTS
* Use as is (Hit Dice times 4.5, plus bonus as shown).
* AD&D tended to use more monsters that were easier to kill.
TREASURE TYPE
* Use 5e tables
NO. OF ATTACKS: Use as is.
DAMAGE/ATTACK: Use as is.
SPECIAL ATTACKS AND DEFENSES: Varies, DM’s call.
MAGIC RESISTANCE: If any, give advantage on saving throws versus spells.
INTELLIGENCE
* AD&D Non- … 5e INT 10 (+0)
* AD&D Animal … 5e INT 10 (+0)
* AD&D Semi- … 5e INT 10 (+0)
* AD&D Low … 5e INT 10 (+0)
* AD&D Average … 5e INT 10 (+0)
* AD&D Very … 5e INT 12 (+1)
* AD&D Highly … 5e INT 14 (+2)
* AD&D Exceptionally … 5e INT 16 (+3)
* AD&D Genius-level … 5e INT 18 (+4)
* AD&D Supra-genius … 5e INT 20 (+5)
* AD&D Godlike … 5e INT 24 (+7)
ALIGNMENT: As is.
SIZE: As is.
PSIONIC ABILITY: DM’s call, but when in doubt treat as attack causing psychic damage.
SAVING THROW CONVERSIONS
* vs. AD&D Paralyzation, Poison or Death Magic … Constitution save
* vs. AD&D Petrification or Polymorph … Constitution save
* vs. AD&D Rod, Staff or Wand … as per spell effect
* vs. AD&D Breath Weapon … Dexterity save
* vs. AD&D Spell … as per spell effect
Loose rule of thumb…
* abjuration effect … Charisma save
* conjuration effect … Constitution save
* divination effect … Constitution save
* enchantment effect … Wisdom save
* evocation effect … Dexterity save
* illusion effect … Intelligence save
* necromancy effect … Constitution save
* transmutation effect … Wisdom save
Example…
ACHAIRERAI (“ash-air-ee-rye”)
A tall fiendish flightless bird that walks on four stilt-like legs, each unnaturally harder than steel.
(from the AD&D1e Fiend Folio, p. 9)
Description (from the Field Folio text): “The legs are a metallic blue-grey, the body-head a dull scarlet with deep red blotches. The eyes are steel-blue and the wings blue-green. A wide range of crest colours have been observed, the most common being a bright flame-red.”
FREQUENCY: Very rare
NO. APPEARING: 1 to 8
ARMOR CLASS: body AC 12, legs AC 21
MOVE: 40 feet (or 10 feet if reduced to one leg)
HIT DICE: as HD 8 (+5 attack, DC 12 save)
* Body: 40 hp
* Each Leg: 15 hp
% IN LAIR: 5%
NO. OF ATTACKS: 3 (two leg strikes and a bite, “claw/claw/bite”)
DAMAGE/ATTACK: 1d8 piercing (leg) / 1d8 piercing (leg) / 1d10 slashing (bite)
INTELLIGENCE: Average (+0)
ALIGNMENT: Chaotic Evil
SIZE: Large (15 feet tall)
TRAITS
Magic Resistance. Gains advantage on saving throws against spells.
Tall Height. Its body cannot be attacked by melee weapons with a reach shorter than 10 feet. Weapons with 10-foot reach, or ranged attacks, may target the body normally.
Toxic Smoke. If the achairerai loses three legs, it immediately releases a cloud of black toxic smoke to cover its retreat. The cloud covers a 10-foot radius around the creature, giving it heavy concealment. Any creatures caught in the cloud must make a DC 12 Constitution saving throw to avoid taking 2d6 poison damage and suffer disadvantage on Intelligence and Charisma ability checks. Those that make the save only take half poison damage and no ability hindrance. At the end of each turn, a creature suffering Intelligence and Charisma disadvantage may repeat the Constitution saving throw to end the effect.