Underworld Overhaul, Pt. 1: Monster Level Tables

We noted previously that numbers for monsters and treasure, in the dungeon in OD&D, were at their root determined by DM fiat. Now we'll spend a few posts trying to massage what is there into a more formal and evidence-based system.

The first part here is probably the easiest: Re-aligning the monster level tables via some underlying rule. Fortunately, we have the fairly well-justified Equivalent Hit Dice (EHD) values available from our previous Arena/Monster Metrics program, and it is relatively straightforward to turn those into a rule for distributing monsters among the 6 "monster levels" in OD&D. Based on inspection of the original book tables, comparing to our EHD values, and also balancing with various experiments in the Arena program, we propose the following as a pretty good definition (see also on GitHub):

Further below, you'll see a complete listing of underworld OD&D monsters parsed into the six levels according to this metric. Most monsters stay at the same level as they appear in the Vol-3 book (p. 1011), although there are some notable changes. Also, monsters are distributed fairly evenly across the six levels, with around 16 monsters each on average; fewer at Levels 1-2 (about 10 apiece), and more in the catch-all Level 6.

To compare to the book system, you can see an old analysis here, in which shaded rows indicate the original monster tables. The book tables have cutoffs at about EHD 1/2/3/4/6/10. But with better monster metrics, you can see in the link that 3rd level has a big problem with how few monsters it keeps under that rule. If we used that book rule to assess the current expanded monster list, then the problem would remain; only about 9 or 10 monsters at levels 2-3; and an enormous dump of 35 monsters all at level 6. So this strongly indicated that we needed to massage the book pattern a bit for a somewhat better distribution.

In the new list below, the third column in the list below also shows the kill ratio attributable to each monster within a level, based on about 2.4 million combat simulations of 4-man fighter parties assaying into a standard-type dungeon, and encountering bands of monsters at equivalent EHD values. (This was generated with the current Arena program using these command switches: -n=10000 -v -z=4 -rk.) Monsters in each level are sorted by increasing kill ratios, so that the whole list runs uniformly from lower to higher danger.

The kill-ratio organization also allows us to look for outliers at the start or end of a table, which may argue for incrementing or decrementing the EHD value of any monsters "on the bubble", so to speak, and this was done manually in a few cases. (Specifically, EHDs for the following were hand-tuned based on this analysis: Large Spider, Giant Beetle-Bombardier, Hell Hound, Harpy, Hero, Swashbuckler, Gray Ooze, and Zombie.) As one example, the Zombie (at the newly-recognized 2 HD) is inevitably going to stick out as either the strongest Level 1 monster, or the weakest Level 2 monster; we've set the EHD at 2 so it appears at Level 2 and smooths out the tables sizes a bit.

With that in mind, here are the proposed new Monster Level Tables:

#	Level 1 Monsters	Kill %
1	Giant Rat	3%
2	Giant Centipede	4%
3	Kobold	7%
4	Skeleton	9%
5	Goblin	9%
6	Large Spider	9%
7	Bandit	10%
8	Orc	10%
9	Giant Beetle, Fire	13%
10	Stirge	13%
11	Hobgoblin	13%
#	Level 2 Monsters	Kill %
1	Zombie	4%
2	Gnoll	7%
3	Berserker	7%
4	Giant Frog	7%
5	Lizard Man	8%
6	Gray Ooze	10%
7	Giant Ant, Worker	11%
8	Giant Hog	13%
9	Giant Lizard	15%
10	Wererat	16%
#	Level 3 Monsters	Kill %
1	Ghoul	2%
2	Giant Toad	3%
3	Bugbear	3%
4	Giant Tick	4%
5	Giant Weasel	4%
6	Giant Beetle, Bomb.	4%
7	Ogre	6%
8	Werewolf	6%
9	Wight	6%
10	Doppleganger	6%
11	Wereboar	7%
12	White Ape	9%
13	Wraith	13%
14	Shadow	14%
15	Warrior	14%
#	Level 4 Monsters	Kill %
1	Minotaur	3%
2	Werebear	4%
3	Giant Snake	4%
4	Harpy	5%
5	Giant Beetle, Boring	5%
6	Giant Wasp	5%
7	Owl Bear	5%
8	Swashbuckler	6%
9	Weretiger	6%
10	Gargoyle	6%
11	Gelatinous Cube	6%
12	Cockatrice	6%
13	Giant Ant, Warrior	7%
14	Myrmidon	7%
15	Hero	11%
16	Hell Hound	15%
#	Level 5 Monsters	Kill %
1	Troll	1%
2	Hydra, 6 Heads	2%
3	Manticore	3%
4	Displacer Beast	3%
5	Giant Beetle, Stag	3%
6	Giant, Stone	3%
7	Lord	5%
8	Giant Slug	6%
9	Mummy	6%
10	Giant, Hill	6%
11	Giant, Frost	7%
12	Superhero	8%
13	Salamander	8%
14	Minotaur Lizard	9%
15	Invisible Stalker	9%
16	Giant Scorpion	9%
17	Spectre	11%
#	Level 6 Monsters	Kill %
1	Ettin	0%
2	Giant Beetle, Rhino.	0%
3	Giant, Cloud	0%
4	Giant, Fire	1%
5	Hydra, 10 Heads	1%
6	Fire Lizard	1%
7	Roper	1%
8	Umber Hulk	1%
9	Balrog	1%
10	Chimera	1%
11	Golem, Flesh	1%
12	Carrion Crawler	1%
13	Wyvern	2%
14	Phase Spider	2%
15	Giant, Storm	2%
16	Dragon, White	4%
17	Dragon, Black	4%
18	Gorgon	5%
19	Dragon, Green	6%
20	Dragon, Blue	6%
21	Mind Flayer	6%
22	Dragon, Red	6%
23	Ogre Mage	6%
24	Medusa	7%
25	Will-O-Wisp	7%
26	Basilisk	8%
27	Vampire	9%
28	Purple Worm	9%