Some Thoughts on Converting AD&D Monsters to Old School Hack

 Okay, so I'm not quite done with my current design obsession with Old School Hack.  A little while back I posted some ideas for future design projects along this line.  If there's one thing I have little problem doing, it's coming up with new ideas.

Recently I was looking over the D&D classics pdf versions of old modules on DrivethruRPG.  I was pleased to see that they had the old AD&D Desert of Desolation modules.  I ran these for my group using the Chivalry & Sorcery rules back in the day.  We had a lot of fun with them even though my conversion to C&S was pretty rough and ready, especially when it came to AD&D creatures which my game didn't have.

So then I was thinking, hey, what if I convert the Desert of Desolation modules to Old School Hack?  Hmm, well being the the real-world Lawful alignment that I am I immediately knew that I needed A System for that.  Now, Old School Hack contains many of the general concepts from the old school D&D games, but it's absolutely not one of the "clones".  That means that in some areas a simple conversion rule will suffice but in others I need to actually do a bit of thinking.  Since I want to be able to convert entire AD&D modules the conversion rules need to be quick and easy to apply.

So I jumped right into the first module, Pharaoh, and grabbed one of the first encounter tables.  This table has a wide range of creatures, which makes is a good starting place.  Here is an excerpt:

DESERT OF DESOLATION SPECIAL ENCOUNTER TABLE

Dervish Hunting Party (AC 6; MV12"; HD 4; hp 4d10; #AT 1; Dmg 1-8 + 2; AL LN)

Symbayan Airlancers (AC6;MV12";HD4;hp4d6;#AT 1; Dmg 1-8+2; AL LG) fly overhead in the distance on the
backs of Pegasi (AC 6; MV 24"/48"; HD 4; hp 4d6; #AT 3; Dmg 1 -8/1 -8/1 -3; AL CG

Purple Worm (AC 6; MV 9"; HD 15; hp 54; #AT 1 and 1; Dmg 2-24 (2d12)/1-4; AL N; tail has death poison).

Okay, so we go from 4HD dervish hunters to 15HD purple worms all in the same table.  The main characteristics we will have to convert are these six (well, okay, so the last one is a generic catch-all):

AC
Move
Attack Bonus
Hit Dice
Number of Attacks
Damage
Special Stuff


Let's take them one by one:

AC - In OSH, AC is a roll-over system rather than roll-under and go from 8 (no armor) to 16 (Uber armor).  The AD&D ACs go from 10 (no armor) to -2 or more.  So we can do a rough conversion chart like this:
AD&D = OSH
10 = 8
9 = 9
8 = 10
7 = 11
6 = 12
5 = 13
4 = 14
3 = 15
2 = 16
1 = 17
0 = 18
...etc.

Move - Hmm, well OSH doesn't bother detailing movement rates; the easiest thing is just use the rates from AD&D.

Attack Bonus -  In AD&D character classes get increased attack bonuses with higher levels and monsters get bonuses which rise with the number of their hit dice.  OSH is a very flat system, where characters basically start with and stay with 5 HP and there are almost no increases to attack bonuses.  The easy fix is to go with what's in OSH, which basically means ignoring bonuses for all NPCs and monsters.

Hit Dice - The easy way is one AD&D HD = one OSH Hit Point; there, that was easy.

Number of Attacks - what it says.

Damage - Okay, here's where it gets tricky.  In AD&D, damage has high granularity ranging from 1d2 to multiple dice of varying sizes such as the 2d12 for the purple worm above.  OSH attacks, however, deal "wounds" which are low-granularity lumps of damage.  OSH damages is more "yes or no damage" versus "high or low damage".  Plus, OSH damage comes in three classes, normal (one point of damage), heavy (two points), and very heavy (two points, or three if you roll well over the opponent's AC).  Wow, this is tough.  We can simplify it down to:
AD&D = OSH
1 damage die = 1 damage
2 damage dice = 2 damage
3  damage dice = 3 damage
4 or more damage dice = 4 damage

This damage conversion approach is very rough and I don't think it will properly reflect the AD&D damage ranges.  A more "accurate" approach would be based on working out the average damage for the creature's attack and using that as the value for conversion to OSH damage.  However AD&D has many, many different dice combinations for damage and I really don't feel like making up a huge chart of them all and working out all the averages.  The above chart is much simpler.

So then, back to the encounter table above.  Applying the conversion rules above we get:

OSH DESERT OF DESOLATION SPECIAL ENCOUNTER TABLE

Dervish Hunting Party (AC 12; MV 12"; HP 4; #AT 1; Dmg 1; AL LN)

Symbayan Airlancers (AC 12; MV 12";HP 4; #AT 1; Dmg 1; AL LG) fly overhead in the distance on the backs of Pegasi (AC 12; MV 24"/48"; HP 4; #AT 3; Dmg 1/1/1; AL CG

Purple Worm (AC 12; MV 9"; HP 15; #AT 1 and 1; Dmg 2 / 1; AL N; tail has death poison).