There are many resources online how to convert creatures and people from D&D to various systems but I’m gonna try to make up some ways to convert to D&D.
This one ended up a lot more 5e-specific than I had intended. I wanna make a separate post later for old-school D&D.
Ideally, we want unambiguously deterministic methods of getting the D&D stats. Not to eyeball on the fly, “this looks like it could be 15 AC maybe”. No. We want to put numbers in and get numbers out.
This “determinism” is the most important quality for on the fly conversion. When doing conversion ahead of time you can afford to hand-tweak a little (“This art seems like it’s wearing plate”). So if you are converting ahead of time you can use a lot more subjectivity and common sense and just bring the general idea over in a way that makes sense. But when the players run into a particular monster that you’ve placed in the dungeon, it’s too late for you to make subjective decisions about stats for it then.
Accuracy is important but of secondary importance.
There are three kinds of accuracy in this context.
Is this creature like a dragon? Like a bear? What would be the outcome in D&D of the player characters facing a D&D dragon or a bear?
OK, so it’s not similar to any known creature. What would be the outcome of the conflict as a whole in the other system? What stats in D&D would lead to a similar outcome?
What’s the effect of one attack? The least relevant and interesting kind of accuracy especially if the systems are very different.
With that in mind (especially if you hand-port creatures), I’m gonna be super roughly and coarsely doing some formulas for this first draft of the document. Determinism & simplicity over any of those levels of accuracy.
Many OSR games let you run monsters just from the HD. So does 5e. So if we can get just one number, we could work from there. Now, a lot of these systems are hopefully giving us more than that.
Strength/3, rounded up.
If the monster does have the 1d6 table of random attacks available, use it, it’s one of the most fun parts of FL. The monster hits as a creature of its HD. Damage, 1d6 for each pips. (“Nine dice and two base damage” becomes 2d6.) If there isn’t a supplied 1d6 random attacks table you don’t have to dig one up in the bestiary or w/e. Just use normal attacks.
Add GURPS HP and FP together, and then, for every four points of that, add DR. (So if a creature has DR 2, 11 HP and 11 FP, that would become 11+11+(5×2)=32 HP with armor on, and 22 HP with armor off.)
HP and FP are based on ST and HT normally. If there are two values separated by a slash as in some 3e books, use the bigger value.
You know how in D&D 3e/4e/5e you can turn a score into a mod by substracting ten and halving? Here, instead, subtract ten and double. So 14 becomes +8, 11 becomes +2 etc. Do not go below -8 or above +12.
HT is Cha and Con (except HP), ST is used for damage (and HP), DX is for speed and to hit (both mêlée and ranged), IQ is Int and Wis.
Use straight up when it’s listed in the stat block, but it’s usually not… GURPS damage expressions can be a bit difficult to find. You need to look both in the equipment table and the damage table. If you don’t have those available, just do 1d6 + strength modifier as calculated above.
Read the weapon skill as if it were an ability and use the ability modifier method above (i.e. subtract ten and double, but no worse than -8 or better than +12).
Which is highest:
Do the subtract ten and double as normal (maybe you already did that in a previous step) and then add 10 to that.
Sometimes you get templates instead of a stat block and they can be annoying to use, especially before you know that the only times you need to make any choices are when you see this symbol: “•”. You do not need to go through a template and count up every point. So when it says “Hypnotism (H) IQ-2 [1]-12” that means Hypnotism 12, or +4 for our D&D-hacking purps.