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Converting Immortals Ability Scoresby Bruce Heard
For all creatures that are described in the AD&D(R) Game, standard monsters in particular, simply use the ability scores given in the AD&D Game (don't bother with conversions there). For all the others various conversion systems are possible. One is a simple math conversion of the abilities, regardless of the adjustments they imply. The other, far more complicated, involves rough comparisons between the effects of ability adjustments of the two games.
STRAIGHT SCORE CONVERSION: Simply assume that scores of 1-18 are the same in both games. That leaves 19-25 for the remainder of the D&D Game ability scores, as follows:
19 19-30 20 31-42 21 43-54 22 55-66 23 67-78 24 79-89 25 90-00
Of course the problem with this is that an Immortal with a Strength of 100 used to have a +20 adjustment to both attack and damage scores (not the +7/+14 adjustments of the AD&D Game's maximum Strength of 25).
For your typical PC or monster, this probably doesn't matter much, but if you plan on using the Immortals system with the AD&D Game, these ratings won't work.
AJUSTMENT CONVERSION: In this case, you have to come up with separate conversion tables for each set of ability scores since the AD&D Game offers completely different adjustments for each one.
For Strength, you'd need to average out attack/damage adjustments and compare them to the D&D Game's standard ability adjustment. For Dexterity, you could average out missile attack and defensive adjustments and use that as a basis for the comparison, etc.
For example, the AD&D Game Strength Table shows a +7/+14 for a Strength of 25. The D&D Game offers instead single adjustments ranging from +7 to +14 for various Strength scores. I'd hit that one in the middle and consider the D&D Game equivalent adjustment to be +10. So a Strength rating of 25 in the AD&D Game would be equivalent to a rating of 46-53 in the D&D Game. Beyond this score, we end up with creatures stronger than gods in the AD&D Game. There's no way to avoid that. For scores that would end up above the AD&D Game's maximum of 25, use a new rating, much like fighters with Strengths in the upper 18's! Having established that conversion milestone, we can now develop the first conversion table, as follows:
Strength AD&D Rating D&D Rating 1 1 2 2-3 3 4-5 5 6-7 6-7 8 8-9 9 10-11 10 12-13 11 14-15 12 16 13 17 14-15 18 16 18/01-50 17 18/51-75 18 18/76-90 19 18/91-99 20 18/00 21 19 22 20 23 21 24-27 22 28-32 23 33-38 24 39-45 25 46-53 25/01-25 54-62 25/26-45 63-70 25/46-60 71-77 25/61-72 78-83 25/73-82 84-88 25/83-89 89-93 25/90-94 94-96 25/95-97 97-98 25/97-99 99 25/00 100
Of course, this does mean that you should check all the AD&D Game's greater gods with ability scores of 25. Whenever they come into play, I would suggest rolling percentile dice to find out how far up the 25's these abilities really get. Incidentally, this would really help differentiating lesser from greater gods. Of course, nothing prevents anyone from counting a score of 25/01-25 as a "26", and 25/26-45 as "27" instead -- and so forth all the way up to "35". But at least, the percentile system allows you to quickly roll up stats if you ever need them.
Now all you need to do if follow a comparable route for the other five ability tables!