The core mechanic of OED is the Target 20 roll as summarized below. Some game actions are resolved on an X in 6 chance using a d6 (see Adventuring).
Melee Attacks: d20 + Attack Bonus + Str Modifier + Target AC ≥ 20
Ranged Attacks: d20 + Attack Bonus + Dex Modifier + Target AC ≥ 20
Saving Throws: d20 + Level + Save Mod ≥ 20
| Save Type | Save Mod |
|---|---|
| Spells | +0 |
| Breath | +1 |
| Stone | +2 |
| Wands | +3 |
| Death | +4 |
Ability Checks: d20 + Level + Modifier ≥ 20.
Thief Skills: d20 + Thief Level + Dex Modifier ≥ 20.
Learn/Research Spell: d20 + Wizard Level + Int Modifier ≥ 20.
Scale: OED uses the below scale for dungeon combat. For battlefield scale see Book of War.
| Unit | Scale |
|---|---|
| 1 round | 10 seconds |
| 1 turn | 1 minute |
| 1" on table | 5 feet or 1"pace" |
Movement: All player races move at a base rate of 60' per round, equivalent to a light jog. A fast jog is double this (120'). A run is triple (180'), sustainable for a 1 Con + Bonus minutes. A full sprint would be (240'), lasting for 1 plus Con + Bonus rounds. Walking is half (30').
Time: Assume that the amount of time spent searching and exploring is analogous to actual time spent. This will result in checking for wandering monsters every 15 minutes of real time, adding a check for long searches (such as for secret doors, etc.).
Lighting: Torches and lanterns shed good light out to 30', and dim light out to 60'. In dim light one can make out basic shapes and/or glowing eyes.
Infravision: Infravision allows one to see (most) creatures in darkness clearly out to 1/2 the specified distance (30') and dimly out to the full listed distance (60'). It is spoiled when looking directly at a torch/fire source.
Darkness/Blindness: Creatures unable to see suffer a –4 penalty to AC and melee attacks. They cannot make ranged attacks (see errant shots below), move at half speed, and suffer –1 to morale.
In OED a number of tasks are resolved with a d6 roll. The chance for success is listed as X in 6, and can be modified a number of "pips" (numbers) up or down by circumstance or race/class ability.
| Task | Chance | Modifiers/Notes |
|---|---|---|
| Climbing | 2 in 6 | + Dexterity Bonus; - Encumbrance Level (0-3); +2 for using a rope. |
| Detection | 2 in 6 | + Intelligence Bonus |
| Doff Armor | 3 in 6 | Chance per round, Leather and Chain Only |
| Force a Door | 2 in 6 | + Strength Bonus: Locks/Bars -2, Reinforced Gates -4, Solid Metal -6 |
| Swimming | 2 in 6* | + Strength Bonus; -# of Stone carried; +/-1 for water conditions |
| Tracking | 4 in 6** | +/- 1 small or large parties, inclement weather, and the like |
* Fighters and Thieves gain a +2 as noted in their class description, for a base 4 in 6 chance.
** Fighters with the Tracking feat only
Climbing: When climbing you move at half speed on a successful check. A failed check is stalled movement. If you fail the roll by 2 pips or greater you fall.
Climbing with Rope: Grants a +2 to the roll and allows one to move downward at up to 4x speed. A sharp jolt while on the rope results in a 1 in 20 chance for the rope to break.
Hauling with Rope: A character can haul a load safely up or down a rope equal to their Strength score in Stone. A sharp jolt while on the rope results in a 1 in 20 chance for the rope to break.
Detection: Used to find a secret door, listen at a door, discover a trap, etc. Listening can not be done while wearing a helmet.
Doff Armor: It takes 1 round (each) to remove a shield, helm, or backpack. Leather and Chain armor have a 3 in 6 chance per round. Plate requires 3 turns to remove with assistance.
Force a Door: Limited to one character per door panel. Special tools (ropes, ram, etc.) will allow the summation of all character's Strength modifiers.
Jumping: A running jump will allow a man sized creature to cover a distance of d6×3 feet, + Strength Bonus, - encumbrance level. Half again for a long jump, half again for a vertical jump.
Swimming: When swimming you move at half speed on a successful check. A failed check is stalled movement. If you fail the roll by 2 pips or greater you sink (see Drowning/Suffocation). You may swim at full speed with a -2 to the check.
In general, all spells and harmful effects should require an attack or allow some form of saving throw to either negate the effect or result in half effect/damage.
Diseases: When exposed to a disease hazard, a Save vs Breath will avoid Strength damage for the day. Recovery requires bed rest.
- Rat Plague: –1d12 Strength per day. Recover 3 Strength per week of rest.
- Tick Fever: –1d6 Strength per week. Recover 1 Strength per week of rest.
- Leech Fever: –1 Strength per day. Recover 3 Strength per week of rest.
- Mummy Rot: –1 Strength per day. Recovery (See Magic Diseases below).
Magic Diseases: Magical disease such as Lycanthropy and Mummy Rot can be removed via the Remove Curse spell in place of the Remove Disease spell.
Drowning/Suffocation: You have a 50% chance each round without air to suffer 1 point of damage (1d3 if surprised). You also suffer 1d6 damage per Turn (minute) without air. Damage is doubled if surprised.
Falling: Suffer 1d6 damage per 10 feet fallen (Max 50d6). A successful Save vs Stone halves the damage. Damage may be modified by 1 or 2 pips per die based on landing surface.
Level Drain: Characters are allowed a Save vs Death to avoid Level Drain effect. If the save is failed, XP is set to the midpoint of the prior level. Current and Max HP are reduced proportionally (divide by original level), possibly resulting in death at 0hp. The number of lost HP should be noted and returned if/when the level is re-achieved.
Poison: Unless otherwise specified, a failed save vs poison means incapacitation, then death in 1d6 rounds. Normal animals may have delayed or non-fatal effect (HP damage, Incapacitation, etc.)
Surprise: Parties are surprised on a roll of 2 in 6. If the foes are unseen (hidden, invisible, elven cloak, etc.) the chance is increased by 2 pips. If the foes are silent (motionless, undead, moving silently, etc.) the chance is increased by 2 pips. A warning (the presence of light, noisy action, magic detection, etc.) negates the surprise roll. A party with the advantage of surprise gets a free round of action before initiative is rolled.
Initiative: Roll a d6 per side; high rolling side goes first; ties go to players. Players declare and resolve actions around the table in order.
Actions: On their turn, characters can move and then take an action, in that order. The following actions disallow movement during that round:
- Casting a Spell
- Set a Pike
- Retrieve a Scroll
- Use a Retrieved Scroll
- Retrieve and Use a Potion
- Ranged Attacks (unless a weapon thrown in the same direction as the character's movement)
Fleeing: A creature can flee combat and does not draw a free attack at its back, but does suffer the penalty for attacks to the rear.
Withdraw: In combat a creature can move up to half their speed without turning their back on the enemy, preventing an attack against the rear flank.
Critical Hits: In most cases, a natural 1 is an automatic miss, and 20 an automatic hit (DM rules on impossible cases). The former may give a chance for a fumble; the latter does double damage (roll twice with bonuses).
Combat Modifiers: The following situational modifiers may be added to attack rolls.
- Attacks to the target's rear: +2 to hit & To hit roll ignores shield AC.
- Attacker is silent or unseen: +4 to hit.
- Target is stunned, stationary: +4 to hit.
- Target is helpless: +6 to Ranged attacks; Automatic Melee attacks for full damage.
Shots at Groups: When firing at a specific individual in a crowd, if the attack roll (d20 + To Hit + Dex Modifier + Range/Attack Modifiers; but not AC) is less than 10, it misses the (man-size) target.
Errant Shots: Ranged attack rolls (as above) below 10 miss the target, but may hit another nearby (determined randomly). Errant, fumbled, or random attacks into groups are assessed with a raw d20 roll that ignores attacker skill and range modifiers.
Grappling: Roll a melee attack, if succesful, then roll opposed Grapple (Strength) checks (d6 + Str bonus; equal to 1/2 HD for monsters). Failure means the target can act normally, success means the target is immobilized, or pushed up to ten feet. The target must win a Strength check on their turn to break free.
Subdual Attacks: Blunt, wooden weapons can optionally be used to attack for nonlethal damage. If the attack reduces the target to 0 Hit Points, it is knocked out. Damage from critical hits becomes lethal, regardless of attacker's intent.
Zero Hit Points: When a character reaches or goes below 0 Hit Points, they must immediately make a Save vs Death. Failure means the character is Dead. Success results in the character stabilizing at 0HP, waking in 1d6 turns, in an incapacitated state. Any healing (returning the character to 1 HP) will restore normal activity.
Natural Healing: Creatures regain a number of Hit Points equal to Level (Hit Dice) + Con Bonus per week of rest and comfort. The creature can not undergo other meaningful activities.
Magical Healing: The only common source of magical healing in OED are potions of healing and potions of mithridate (neutralize poison) which are available from Wizards and NPC Alchemists. There is no common source of regeneration, resurrection, or restoration magic.
Character Death: Players can bring in a new 1st level character at the completion of current combat/activity.
OED makes the below spells available to the Wizard class. Spell attributes and effects have been standardized and are available in the OED Book of Spells.
| Level 1 | Level 2 | Level 3 |
|---|---|---|
| 1. Charm Person | 1. Continual Light | 1. Clairaudience |
| 2. Detect Magic | 2. Darkness | 2. Clairvoyance |
| 3. Hold Portal | 3. Detect Evil | 3. Dispel Magic |
| 4. Light | 4. Detect Invisibility | 4. Fireball |
| 5. Magic Missile | 5. Invisibility | 5. Fly |
| 6. Protection from Evil | 6. Knock | 6. Haste |
| 7. Read Languages | 7. Levitate | 7. Hold Person |
| 8. Read Magic | 8. Locate Object | 8. Infravision |
| 9. Shield | 9. Magic Mouth | 9. Invisibility Sphere |
| 10. Sleep | 10. Mirror Image | 10. Lightning Bolt |
| 11. Phantasmal Force | 11. Protection from Missiles | |
| 12. Pyrotechnics | 12. Protective Sphere | |
| 13. Read Minds | 13. Rope Trick | |
| 14. Strength | 14. Slow | |
| 15. Web | 15. Suggestion | |
| 16. Wizard Lock | 16. Water Breathing |
| Level 4 | Level 5 | Level 6 |
|---|---|---|
| 1. Charm Monster | 1. Animal Growth | 1. Antimagic Sphere |
| 2. Confusion | 2. Animate Dead | 2. Control Weather |
| 3. Dimension Door | 3. Cloudkill | 3. Death Spell |
| 4. Extend Spell | 4. Conjure Elemental | 4. Disintegrate |
| 5. Fear | 5. Contact Other Plane | 5. Geas |
| 6. Hallucinatory Terrain | 6. Feeblemind | 6. Invisible Stalker |
| 7. Ice Storm | 7. Hold Monster | 7. Legend Lore |
| 8. Plant Growth | 8. Magic Jar | 8. Lower Water |
| 9. Polymorph Other | 9. Passwall | 9. Move Earth |
| 10. Polymorph Self | 10. Telekinesis | 10. Project Image |
| 11. Remove Curse | 11. Teleport | 11. Reincarnate |
| 12. Wall of Fire | 12. Transmute Rock to Mud | 12. Stone to Flesh |
| 13. Wall of Ice | 13. Wall of Iron | |
| 14. Wizard Eye | 14. Wall of Stone |
- Spells generally need line-of-sight to their target point.
- All spells permit saving throws for anyone directly or negatively affected by them (half effect for damage type spells).
- Concentration (when required) permits ½ speed movement, but no attacks, spells, or taking damage.
- “Man-size” creatures are those of 1 HD (base, without class levels).
- Low-level detection spells (up to 3rd level) are generally blocked by 3 feet of dirt or wood, 1 foot of stone, 1 inch of metal, or a thin sheet of lead. Higher-level spells are blocked only by special protections.