Monsters vs. Spells: The Arena OD&D simulator is purely a 1-on-1 man-vs-monster combat (as of this writing); monster of high Hit Dice value need to be cut down one d8+1 sword blow at a time. Of course, an associate wizard with proper spell selection could radically simplify the process, but this depends on the monster involved. For low-level mobs, then area-effect spells could clear the room, as with sleep, fireball, or cloudkill. But for single high-powered monsters, then a targeted save-or-submit spell like charm or hold monster or disintegrate could end the fight immediately. While I'm happy to use the fighting EHD as a baseline for lesser monsters, at the upper end things get a lot more hazy about whether spell options should really be the major consideration.
Hydras: Hydras with 6-8 heads are listed in OD&D monster table Level 5; and Hydras with 9-12 heads are on the Level 6 table. As usual, in each case I picked one representative type for my Arena input. The 6-headed Hydra is reasonably given EHD 11, about double the HD 6. But the 10-headed Hydra came out at EHD 44, unprecedentedly more than 4 times the HD of 10, and at least twice the EHD of the next-lower monster on the list.
So: Hydras are dangerous, really dangerous. The fact that it can make so many attacks, possibly up to 10 dice of damage per turn, and those attacks are at a high hit-level of 10 HD or more (even after losing some heads), really makes it a whirlwind of death to a lone fighter. A high-level fighter with higher abilities, with magic plate, shield, and sword of multiple bonuses, will have a much better time against it (both in avoiding more of the bite attacks, and getting average damage above 6 points means you can probably reduce the head count by at least one each round). The AD&D Monster Manual made a major rule adjustment: no more than 4 heads can attack any one creature; but that limitation does not appear in OD&D. Perhaps that restriction is both reasonably balanced and also intuitively realistic (in that some limitation should prevent innumerable heads from all getting tangled up in one space, just like any other monster is limited in number of attacking bodies they can throw against a PC).
If we think about including spell-casters in our game, then subjectively this could be altered further downward, but the exact level depends keenly on adjudications regarding Hydras, not explicated in the rules text for any edition. For example: I've seen many D&D games get quagmired when the DM drags out a Hydra, a PC hits it with a spell, and then no one has any idea how rule on it. For example: Say a fireball is rolled for 20 points of damage; does this apply one time to the whole Hydra (killing at most 3 heads), or area-effecting all the heads separately (killing them all)? Assume that a charm monster spell is cast at a Hydra: Does it affect a single head (causing it to attack other heads), or does the entire Hydra become subdued with a single saving throw? What about disintegration? Would a cloudkill or death spell consider each head as separate 1-HD creature (if "yes", then the hydra is defeated; if "no", then the hydra is immune).
Now, personally I would rule that in the spirit of the thing, heads are never treated as separate creatures for the purpose of spell effects, and I hope that most of us would agree (so: average fireball gets maybe 3 heads; sleep, cloudkill, and death spell have no effect; but charm, hold, and disintegrate stop the whole monster). On the other hand, I have seen some DM's go the other way with that; and more worrisome, it's a little unpredictable because it seems that many of us haven't thought it through in advance. The exact risk level, and hence XP value, of Hydras depends a lot on this off-book decision-making. But clearly they are dangerous top-of-the-food-chain monsters.
Purple Worms: I set my simulator to do a binary search of fighters between levels 0 and 50 to find the best match for any given monster (with 10,000 fights between random monsters and fighters at each level). Initially it had a maximum level of 20, but I found a number of monsters bumping up against that ceiling. We can find one monster that will bump up against any ceiling we specify, that is, it has undefinable (effectively infinite) EHD in this model; the Purple Worm (HD 15).
The save-or-die poison tail sting is bad enough, but as Vol-2 rightly asserts, "its mouth is the more fearsome weapon". With a roll of more than 4 points over the to-hit number, the purple worm swallows its prey into its gullet, where it dies automatically 6 rounds later (and is irrecoverably disintegrated 6 rounds after that). Note that for our baseline fighter in chain & shield, AC 4, the worm effectively always hits (20 − 15 − 4 = 1 to hit!), and thus swallows on any roll of 6 or more -- 75% chance on any attack to instantly take the fighter out of the fight, of any level (for a total probability of 99.99% to win such a fight). Or let's compare to the case of a fighter fully equipped with magic +5 plate, +5 shield, and +5 sword (assume average Dexterity), AC −8; so the worm still hits almost half the time (20 − 15 − (−8) = 13), and still swallows on an 18+ roll -- 15% chance to end the fight on any attack, regardless of opponent level (overall, the worm is favored at 57% to win a fight against any fighter of any arbitrarily high level in this model).
As an aside, we see that the Vol-2 rule clause of "or 100% [natural 20] in any case" is effectively redundant, as any character of any possible AC value is swallowed on some roll of less than 20 anyway.
In OD&D, there is simply no mechanism mentioned such that a swallowed character can escape on their own (excluding friends presumably killing the beast and cutting them out in time), so we know that the solo fighters in the Arena will definitely be destroyed as soon as a Purple Worm swallows them. The later AD&D Monster Manual added a remedy to this situation, by stating, "Note, however, that a creature swallowed can try to cut its way out of the purple worm's stomach. The inner armor class of the 'worm is 9, but each round the creature is in the worm it subtracts 1 from the damage each of its attacks does." Now, at first glance this rule might seem reasonable to us (a nod to Jonah), but personally it always gives me indigestion. Notice that the character swallowed is under no threat of attack or damage while inside, and the inner AC of 9 is the easiest possible target in the game (a high-level fighter should automatically hit it, much like the Purple Worm automatically hit him in the first place). So in practice it turns out that, under this rule, the safest place to fight a Purple Worm is inside it. I've seen fights degenerate into a comical sequence of: fighter gets swallowed; fighter cuts way out; fighter gets swallowed again; rinse and repeat, etc. With this rule, the Swallow attack is not so much a feared ability, it's actually the best defense against the Purple Worm itself. For this reason, I am very much opposed to the AD&D rule, and keep the special ability as a Doom-That-Came-To-Swallow kind of menace. Bring lots of friends to cast missiles and cut you out, or don't face off against a Purple Worm in the first place.
The Arena simulator reflects this pure-OD&D adjudication, and so the Purple Worms are favored to eat up any single fighter regardless of level; and thus the EHD is effectively undefined for this type. Perhaps we should use other monsters as a guidepost and say that it likely tops out at about triple HD/XP?
Conclusions: Taken as printed in our Arena output chart, we could take the listed EHD's as the exact number to use in place of HD when we look up awards in the XP table; or more simply using Vol-1 rules, compute EHD × 100 in each case. But granted that even this is a rough model, and that some subjectivity exists for consideration of multiple party members and spell-casting, perhaps the simplest and most honest thing to say that at best we may have the right magnitude of multiplier by which to adjust XP. That is: either double or triple XP, as shown under the "Multiplier" column in the table, and not pretend to more precision than that.
Even this has some interesting discoveries, I think, like: Wights, wraiths, mummies, lycanthropes, and even giant constrictor snakes probably don't deserve any XP bonuses. Basilisks and Medusae are probably reversed in the tables in terms of danger level. Most monsters likely don't deserve more than a single doubling of XP, with only the rarest of exceptions (even Vampires with their half-dozen abilities may only deserve one doubling, according to this).
Now, while OD&D Vol-1 didn't make any mention or awareness of the need to adjust monster XP for special abilities (actually, the Troll example explicitly avoids making any such adjustment), the need must have been rather quickly noticed, because a D&D FAQ article in Strategic Review #2 (Summer 1975) did bring up the topic:
Experience: ... For purposes of experience determination the level of the monster is equivalent to its hit dice, and additional abilities add to the level in this case. A gorgon is certainly worth about 10 level factors, a balrog not less than 12, the largest red dragon not less than 16 or 17, and so on. The referee's judgement must be used to determine such matters, but with the foregoing examples it should prove to be no difficulty.
In broad strokes, this is the same method we recommend here (add some integer number to the Hit Dice to adjust for special abilities, before computing XP, without any extra table or more complicated calculations being made). How much do we agree with those suggestions? The Strategic Review article probably undervalues those top-level monsters by quite a bit. For Gorgons it suggests "about 10 level factors", but here we think 15 is better. For a Balrog it says "no less than 12", and strictly speaking we agree with the inequality statement, a value of 22 being reasonable. For the largest Red Dragon it says "not less than 16 or 17", and the Arena simulator would suggest that a value of even 45 would not be out of the question for eldest type.
Hopefully this gives some additional perspective and confidence when assigning XP under the Original D&D (Vol-1) system. Perhaps equally important, it was a great motivation to crawl into the guts of the OD&D monster special abilities, and be forced to think about the specifics of adjudicating each, given the level of complete discipline and specificity that our computer programming framework forces upon us.
Eaten any thoughts? Devoured any intellects?