OED uses the monsters presented in Vol II with the adjustments below.
Hit Dice: All monsters use a d6 for their base Hit Dice, which is used to calculate combat abilities.
Equivalent Hit Dice: Monsters in OED have an Equivalent Hit Dice value that takes into account their special abilities and relative power. This is used to determine monster encounters and XP rewards.
Attacks: Monster attacks are resolved by d20 + Hit Die + Target AC ≥ 20. Monsters get a single attack per round unless otherwise noted.
Monster Damage: Monster damage is rolled on a 1d6, unless otherwise noted.
Monster Strength: If necessary, you can assume a Monster’s Strength modifier is equal to ½ its Hit Die, and their carry capacity in Stone is (HD*2)+10 (Multiply by 2 or 3 for quardrapeds, etc).
Gaze Attacks: Creatures facing beings with Gaze Attacks must make a Save each round unless precautions are taken.
Grab/Grappling: Roll a melee attack, if succesful, then opposed Grapple (Strength) checks (d6 + 1/2 HD vs d6 + Target Strength Bonus). 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.
Languages: It is assumed than an intelligent creature has a base 1 in 6 chance to speak common.
Magic to Hit: For every 4 natural Hit Die a creature possesses, it is capable of harming creatures that normally require +1 magic weapon to hit.
Mental Attacks: Undead, golems, slimes, plants, and other non-intelligent creatures are generally immune to such effects, unless specifically targeting that type.
Chimera/Gorgons: Each round roll a 2d6, a result of 7+ means the creature will use its breath weapon. It can use its breath weapon a total of 3 times per day.
Dragons: Divide the Dragon’s HP by 3 to determine its Hit Dice. Use this Hit Dice value to determine Attacks and Saves. Each round roll a 2d6, a result of 7+ means the creature will use its breath weapon. It can use its breath weapon a total of 3 times per day. A Dragon’s breath weapon does 1 die of damage for every 6 hit points it currently possesses. Dragons can force a morale check on NPCs within 75 feet. They have a 2 in 6 chance to detect creatures within 75 feet. Dragon subdual rules are not used.
Goblin-kin: Goblins, Orcs, and Kobolds suffer a -4 to attack, and -1 to Morale in full daylight.
Giants: Giants can throw a stone every other round at ranges of 100’ to 140’. Giant melee attacks are 1d10 +Str bonus. Giant garb is equivalent to Leather armor on top of naturally tough skin (+3 AC).
Horses and Mounts: Only warhorses are usable near combat, fire, smells, etc. (others auto-fail morale). The normal load value indicates the maximum load carried at full-speed; up to double this amount can be carried at half-speed. The normal load values are set at 1/3 of the equine’s body weight.
Slimes: Successful attacks by these creatures will cause metal to corrode instantly. Metal weapons used against them will corrode on a successful attack. Wood is consumed at too slow a rate to affect combat.
Undead: Undead are (mostly) immune to backstabs, critical hits, death effects, and mental attacks.
Skeletons: Skeletons are 1 Hit Die monsters.
Vampires: Vampires can summon 1d10 Rats (AC 7, Mv 75’, HD 1/6 (1HP), Att +0, Dmg 1) or 3d6 Wolves (AC 7, Mv 90’, HD 2+2, Att +1, Dmg 1d6).
Zombies: Zombies are 2 Hit Die monsters.
Within the dungeon, there is a 2 in 6 chance a monster is present in any unallocated space (See Monster Determination below).
Checks: Make a 1 in 6 check for wandering monsters using the below guidelines:
If a wandering monster result is indicated, use the Monster Determination information below. Multiple parties of monsters may attack one another. On a near miss (result of 5), give a hint that monsters are present (ex: distant monster noise).
To determine the type of monster encountered, roll a d6 and consult the result against the level of the dungeon (depth) in question. This will give you the “Threat Rank” (I - VI) of the monster, which you can then look up the appropriate (Equivalent) Hit Die range.
| Dungeon Depth | d6 Result | Threat Rank | Monster (Equivalent) Hit Dice |
||||||
|---|---|---|---|---|---|---|---|---|---|
| 1 | 1-4 | 5 | 6 | I | 1 HD | ||||
| 2 | 1-2 | 3-4 | 5 | 6 | II | 2 HD | |||
| 3-4 | 1 | 2 | 3-4 | 5 | 6 | III | 3-4 HD | ||
| 5-7 | 1 | 2 | 3-4 | 5 | 6 | IV | 5-7 HD | ||
| 8-10 | 1 | 2 | 3-4 | 5-6 | V | 8-10 HD | |||
| 11 | 1 | 2 | 3-6 | VI | 11+ HD | ||||
| Threat Rank | I | II | III | IV | V | VI |
When stocking a dungeon, the numbers of monsters appearing is dependent on the relation between the Equivalent Hit Die of the creature and dungeon level/depth (presuming a traditional party of 4)
| EHD vs Dungeon Level | No of monsters appearing |
|---|---|
| Dungeon level is greater by 3 or more | (1d4+1) or ~2 3/4 party size |
| Dungeon level is greater by 1 or 2 | (1d4+1) or ~ 1 3/4 party size |
| Dungeon level and EHD are equal | 1d4+1 or equal to party size |
| EHD is greater by 1 or 2 | (1d4+1) or 1/2 party size |
| EHD is greater by 3 or more | (1d4+1) or 1/4 party size |
For a wandering monster encounter, cut the above number in half.
For a more detailed breakdown of this formula and approach, see: Underworld Overhaul, Pt. 4: Monster Numbers Appearing
This is a measure of monster threat that can be slotted into monster determination formula (in place of the Hit Die) for a more accurate distribution of challenges.
| Monster | EHD | Monster | EHD | Monster | EHD |
|---|---|---|---|---|---|
| Rat | 0 | Swordsman | 3 | Giant Rhinoceros Beetle | 10 |
| Aquatic Elf | 1 | Wereboar | 3 | Minotaur Lizard | 10 |
| Bandit | 1 | Werewolf | 3 | Morkoth | 10 |
| Berserker | 1 | Wight | 3 | Small Air Elemental | 10 |
| Brigand | 1 | Centaur | 4 | Triton | 10* |
| Buccaneer | 1 | Floating Eye | 4 | Fire Giant | 11 |
| Dervish | 1 | Gelatinous Cube | 4 | Giant Fish | 11 |
| Dwarf | 1 | Giant Crab | 4 | Giant Shark | 11 |
| Elf | 1 | Giant Sea Spider | 4 | Giant Squid | 11 |
| Giant Centipede | 1 | Giant Warrior Ant | 4 | Gorgon | 11 |
| Giant Fire Beetle | 1 | Giant Wasp | 4 | Guardian Naga | 11 |
| Giant Rat | 1 | Gray Ooze | 4 | Magician | 11 |
| Giant Sea Horse | 1 | Harpy | 4 | Medusa | 11 |
| Gnome | 1 | Hell Hound | 4 | White Dragon | 11 |
| Goblin | 1 | Hero | 4 | Black Dragon | 12 |
| Hobgoblin | 1 | Ochre Jelly | 4* | Cloud Giant | 12 |
| Homunculus | 1 | Ogre | 4 | Ettin | 12 |
| Jackal | 1 | Seer | 4 | Trapper | 12 |
| Kobold | 1 | Wraith | 4 | Umber Hulk | 12 |
| Leprechaun | 1 | Dryad | 5 | Black Pudding | 13* |
| Light Horse | 1 | Gargoyle | 5 | Fire Lizard | 13 |
| Merman | 1 | Giant Boring Beetle | 5 | Catoblepas | 14 |
| Nixie | 1 | Giant Crocodile | 5 | Efreeti | 14 |
| Nomad | 1 | Giant Octopus | 5 | Enchanter | 14 |
| Orc | 1 | Giant Poisonous Snake | 5 | Green Dragon | 14 |
| Piercer | 1 | Lion | 5 | Medium Roc | 14 |
| Pirate | 1 | Portugese Man-Of-War | 5 | Blue Dragon | 15 |
| Pixie | 1 | Rust Monster | 5* | Flesh Golem | 15 |
| Skeleton | 1 | Small Roc | 5 | Small Fire Elemental | 15 |
| Stirge | 1 | Swashbuckler | 5 | Small Water Elemental | 15 |
| Veteran | 1 | Weretiger | 5 | Water Naga | 15 |
| Wolf | 1 | White Ape | 5 | Mastodon | 16 |
| Caveman | 2 | Bear | 6 | Mind Flayer | 16 |
| Crocodile | 2 | Cockatrice | 6 | Storm Giant | 17 |
| Dire Wolf | 2 | Conjurer | 6 | Ten-Headed Hydra | 17 |
| Dolphin | 2 | Displacer Beast | 6 | Warlock | 17 |
| Draft Horse | 2 | Ghast | 6 | Balrog | 18 |
| Giant Bombardier Beetle | 2 | Giant Constrictor Snake | 6 | Medium Air Elemental | 18 |
| Giant Eel | 2 | Giant Spider | 6 | Ogre Mage | 18 |
| Giant Frog | 2 | Giant Stag Beetle | 6 | Roper | 18 |
| Giant Hog | 2 | Minotaur | 6 | Shambling Mound | 18 |
| Giant Lamprey | 2 | Mummy | 6 | Small Earth Elemental | 18 |
| Giant Leech | 2 | Myrmidon | 6 | Small Sea Monster | 18 |
| Giant Lizard | 2 | Owl Bear | 6 | Tyranosaurus Rex | 18 |
| Giant Tick | 2 | Pungi Ray | 6 | Basilisk | 20 |
| Giant Worker Ant | 2 | Unicorn | 6 | Giant Manta Ray | 20 |
| Gnoll | 2 | Champion | 7 | Medium Fire Elemental | 20 |
| Ixitxachitl | 2 | Djinni | 7 | Medium Water Elemental | 20 |
| Large Spider | 2 | Lammasu | 7* | Red Dragon | 20 |
| Lizard Man | 2 | Sabre-Tooth Tiger | 7 | Sorcerer | 20 |
| Locathah | 2 | Theurgist | 7 | Will-O-Wisp | 20 |
| Medium | 2 | Werebear | 7 | Large Air Elemental | 22 |
| Medium Horse | 2 | Wind Walker | 7 | Purple Worm | 22 |
| Mule | 2 | Yeti | 7 | Spirit Naga | 22 |
| Pegasus | 2 | Giant Slug | 8 | Large Roc | 24 |
| Ram | 2 | Griffon | 8 | Necromancer | 24 |
| Sahuagin | 2 | Hill Giant | 8 | Dragon Turtle | 25 |
| Troglodyte | 2 | Invisible Stalker | 8 | Treant | 25 |
| Warrior | 2 | Phase Spider | 8 | Vampire | 25* |
| Weed Eel | 2 | Six-Headed Hydra | 8 | Medium Earth Elemental | 26 |
| Wererat | 2 | Slithering Tracker | 8 | Wizard | 26 |
| Yellow Mold | 2* | Spectre | 8 | Masher | 28 |
| Zombie | 2 | Superhero | 8 | Large Fire Elemental | 33 |
| Blink Dog | 3 | Thaumaturgist | 8 | Large Water Elemental | 33 |
| Boar | 3 | Troll | 8 | Medium Sea Monster | 33 |
| Bugbear | 3 | Carrion Crawler | 9 | Gold Dragon | 35* |
| Bull | 3 | Giant Scorpion | 9 | Large Earth Elemental | 36 |
| Doppleganger | 3 | Giant Sea Snake | 9 | Lich | 40* |
| Ghoul | 3 | Lord | 9 | Whale | 40 |
| Giant Toad | 3 | Lurker Above | 9 | Large Sea Monster | 45 |
| Giant Weasel | 3 | Manticore | 9 | Titan | 50* |
| Green Slime | 3* | Salamander | 9 | Rakshasa | 55 |
| Heavy Horse | 3 | Stone Giant | 9 | Stone Golem | 55* |
| Hippogriff | 3 | Wyvern | 9 | Beholder | 60* |
| Huge Spider | 3 | Animated Tree | 10 | Iron Golem | 140* |
| Shadow | 3 | Chimera | 10 | ||
| Shrieker | 3 | Frost Giant | 10 |
* Indicates The EHD for this creature is highly situational.
For a more detailed list of appropriate monsters and Equivalent Hit Dice, consult the OED Monster Determination sheet.