One of the great things about classic D&D is that it is very easy to improvise monsters. This can be handy not only for “winging it” in general, but to “convert” adventures written for other systems on-the-fly. On G+, it was suggested that this may not be obvious to beginning DMs, so here are some thoughts on it.
If it matters to interpret anything I say below, this is aimed towards the 1981 version of non-Advanced D&D—often called B/X or Moldvay/Cook/Marsh—simply because that’s my D&D lingua franca. It probably won’t matter.
The first thing to say is: Don’t panic. It’s OK if you mess it up. Firstly because there’s a lot of variability in the game. Once the dice hit the table, a monster that was carefully design to be unbeatable dies in one round and a monster that was carefully designed to be no more than a distraction kills the entire party. Secondly because, if you give the PCs the freedom you should, they can figure out how to handle whatever you throw at them. Thirdly because the only way you’ll get any better is to mess it up first.
Let’s start with HD: Hit dice. A monster’s HD tells you most of what you need to know about it from a mechanical viewpoint. Just think of it as equivalent to character level. It isn’t, but it is close enough. (See “Don’t panic” above.) Ask yourself, what level should this monster be if it were an NPC, and set that as its HD.
If you want to roughly scale the monsters versus your players’ characters, add up the levels of the PCs. A group of monsters with an equal number of HD is—in theory—their match. e.g. For a party of 4 second-level characters this might be one 8 HD monster or eight 1 HD monsters. Generally you don’t want every encounters to be an equal match, so scale back accordingly. (Or, if appropriate, scale up.) But keep in mind that there are a lot of variables that determine the outcome of an encounter. This is no guarantee; simply a guideline. (See “Don’t panic” above.)
I say, don’t worry about trying to factor special abilities into this equation. But, if you want to, go ahead. Just make your best guess.
Also, keep in mind that many low HD monsters have one advantage over a single high HD monster: More actions. Eight 1HD monsters get 8 attacks per round. One 8HD monster might only get one.
Next up is AC: Armor class. The first thing to realize is that hit points (and thus HD) determine a monster’s (or character’s) combat effectiveness more than AC. That means it matters even less whether you pick AC “right”. There are four numbers you need to memorize:
9: The equivalent of unarmored
7: The equivalent of light/leather armor
5: The equivalent of medium/mail armor
3: The equivalent of heavy/plate armor
Note that they’re simply the odd numbers from 3 to 9, which makes it even easier to remember. If you’re unsure which of two categories it should be, pick the even number between them.
Then think about the monster’s dexterity. If you think it would be significantly better or worse than the average PC, then subtract or add 1 to 3 points.
Next, damage: 90% of the time, you can just give it one attack doing 1d6 damage.
If you think the monster should do less than normal, change it to d4. If you think they should do more than normal, change it to d8. If you think they should do a lot more than normal, change it to d10.
I generally like to stick to one attack per combatant. Big monsters who are typically on there own, however, can be well served by multiple attacks.
For saving throws, 99% of the time, you can just use the fighter saving throws using the monsters HD as its fighter level.
If you think the monster is particularly similar to one of the other classes, use it instead. You might decide the monster should have better or worse saves for some reason, in which case you can just use the next higher or lower line on the saving throw table.
For movement: 120 for the same speed as an unarmored PC. Adjust up or down to taste.
For alignment, when it doubt, choose neutral.
For morale: 12 means it never runs away; 2, it never fights. The book tells us 6–8 is average. 6 means it keeps fighting about 42% of the time when morale is checked; 7, 58%; 8, 72%.
Special abilities: If you never give an improv’d monster a special ability, that will be fine. (But I’m betting you’ll come up with some ideas.) Also, don’t worry too much that a special ability may be “too good”. (See “Don’t panic” above.)
There are two schools of thought here. One says that special abilities have more impact when only a few monsters have them. Others feel every monster deserves a special ability, which makes the game richer. Either way, my best advice is to spend some time reading through the monster sections/books to get a feel for what has been done. Keep notes of any you find especially interesting to reference during play. And brainstorm your own.
If you’re converting an adventure on the fly, the text or stats there might also provide inspiration.
Study: You can prepare to improv. While I hope this will convince you that anyone can do a good enough job improvising classic D&D monsters, as with special abilities above, reading the stock monsters or other sources of monsters can help to improve your ability to improv.
Make quick references: Perhaps the most important thing you can do to prepare to improv is to develop some quick reference pages or a DM screen. Having the combat and saving throw tables, AC table, weapon damage table, reaction roll and morale tables, etc. handy can help both improvising monsters and running the game. If, while running the game, there is anything you wish you had handy for reference, jot down a note about it. Then make sure you add it before the next session.
Finally, keep in mind that this game is not about combat. (Combat takes up only 5 of the 64 pages in the 1981 Basic rules booklet.) The things that really bring the game to life can’t be found in the combat stats. Think of a monster as an NPC. They have motivations, needs, desires, quirks, etc. Using reaction rolls and morale rolls (or good judgement by the DM) will mean that a number of encounters never get to combat and those that do sometimes end in surrender or retreat.