This is not an unusual problem for DMs running “old school” games. In 3rd edition D&D, WotC added the challenge rating system to help codify just how nasty a critter was. I judge the results to be mixed, especially at higher levels, but it was a good attempt.
Older versions of D&D don't even have that. DMs are left to “eyeball” the situation as best they can, and the only way to get a good feel for that sort of thing is experience. Things are even worse at 1st level; when most PCs have 1d6 hit points and weapons typically do 1d6 damage, every successful attack has the potential to turn into a devastating alpha strike.
So what's a new DM to do? Here are a few rules-of-thumb you can use to judge the danger of your encounters in an old school game:
- Watch the HD: You can generally judge the toughness of a monster by its hit dice. The more hit dice a critter has, the more hit points it's likely to have, and the better chance it will have to succeed on attack rolls. As a general rule-of-thumb, a group of enemies whose total hit dice is equal to the number of total levels in the PCs and their allies is a strong challenge up until the PCs reach 5th level. (At 5th level, all sorts of wacky things happen, primarily because the PCs gain access to 3rd level spells.)
- Beware the Power of Iteration: A single foe is not as dangerous as a mob. This is due to attack rolls being made on a single d20. Since the probability of any single number coming up is flat, including a 20, rolling more attack dice has a huge effect on combat. This is magnified if you use any sort of “critical hit” rules.
- Save or DIE: There are lots of save-or-die powers in old school monster lists. Most spiders and snakes with venom force a life-or-death saving throw with every successful attack. However, these are not nearly as dangerous as those that force multiple characters to save. For instance, the tarantella's poison might seem a safer choice since it doesn't cause immediate death, but rather a spastic dance. However, anyone who sees someone doing this dance must then save vs. Spells or they'll start dancing, too. A single successful bite can potentially wipe out the entire party!
So, with all this in mind, what can the DM do to provide a little breathing room for both the dungeon and the players without having to worry constantly about building “killer” adventures? One trick I use are “get out of jail free” options. Things like my shields shall be splintered rule allow PCs to ignore one hit, giving them a bit more surviveability. Notice that there's a price for this, however. Squirming over these sorts of choices is part of the fun of old school gaming. The heroes in my Labyrinth Lord game recently found a potion that will restore all hit points, neutralize nearly any poison, and undo effects like paralyzation and blindness. Unfortunately, it also switches your sex if you drink it.
If you're a player, remember that the same iterative power that works for the monsters can work for you as well. Hirelings and henchmen can go a long way towards evening the odds. Don't underestimate the usefulness of clerics, either. Having a few extra hit points in your back pocket, that can be rushed to any member in the party as needed, is a powerful equalizer.
Finally, remember that there are however many of you, and only one DM. Your combined cleverness can almost always trump any idea the DM has come up with. Yeah, clever planning didn't work for Biffy, but lateral thinking is a potent force multiplier that literally has no limit in the sorts of problems it can overcome.
Photo credits: cheesy42.