I always been a fan of 1st edition Battlesystem. Namely because out of all the miniature wargame ruleset for DnD that have been produced I feel it is the one that operates the most seamlessly with ADnD 1st edition. As well as intregrating individual heroes well. There is nothing like seeing the true power of a fireball when you chuck it on a incoming formation of orcs. With makes even more nice that there are notes that tell you how to make it work for plain DnD namely the Mentzer's BECMI line.
I am probably going to use it for an upcoming game using Swords & Wizardry and my Majestic Wilderlands. Now the original Battlesystem used THACO and descending AC to calculate how much damage is done each round. So how do I use it with a retro-clone that has Ascending AC and To-Hit bonus?
Normally it works like this.
2d6+ THACO-AC + modifiers.
I played around with the numbers and found this works
2d6+Ascending AC-To Hit Bonus+modifiers.
So if you have a THACO of 17 and trying to AC 8 the original formula will give you +9
17-8 = 9
With Swords and Wizardry, d20 or any other retro clone using Ascending AC it would work like this.
A THACO of 17 means the character has a +3 to hit bonus as AC 0 in Ascending AC is 20.
AC 8 means AC 12 in Ascending AC so the formula is now
The rest of the Battlesystem rules with dice of damage, modifiers, even the unit stats work pretty much as is with anything from ODnD to 3.5.
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