GM: You’re up, Randy! The goblin stands in front of you, clutching its spear and snarling - what do you do?
Randy: I step forward and aim for it’s neck with my blade!
GM: Great, roll to see if you hit!
…
GM: The other goblin didn’t like that, Randy! It thrusts its spear at you - what do you do?
Randy: I pull my shield up and try to swat the spear away!
GM: Great, roll to see if you defend!
How is the game just not played this way by default? It just makes so much sense!
Luckily, smarter minds than me have figured it out for their own games, and so we can take it and use it here in Basic D&D too.
And good news, converting Basic D&D with Ascending Armor Class (such as Old-School Essentials) to feature player-facing defense rolls is pretty simple:
AC Becomes DEF
In order to determine the Defense Bonus (DEF) that PCs will add to their rolls to defend against NPC attacks, we start with Ascending Armor Class (AAC):
AAC - 10 = DEF
Using this formula, we can determine the following DEF values for armor as follows:
Once we have these values, the PCs will have no need for AC anymore, and can simply make note of their DEF (DEX + the DEF of worn armor). A PC with 8 Dexterity (-1 DEF) and wearing leather armor (+2 DEF) will have a total of +1 DEF.
As always, bonuses from magic armors still apply like normal. A PC wearing +1 chain mail will have an additional +1 DEF, and so on.Because DEF is a player-only stat, we're done here! No need to figure out DEF for NPCs, they'll get an Attack Rating instead.
Attack Bonus Becomes Attack Rating
Our next step is to convert NPC attack bonuses into a static Attack Rating (AR), kind of like Armor Class - but for attacks. This acts as the target number for PC defense rolls.
To determine an NPC's Attack Rating, we'll use the following formula:
12 + Attack Bonus = Attack Rating
Why 12?
Because 12 takes into account our scale where unarmored starts at +0, and also ensures we include the target number itself - not just the numbers greater than the target number.
With this formula, we can easily determine an Attack Rating for any NPC we have an Attack Bonus for: 12 + Attack Bonus. A goblin has an Attack Bonus of +0, therefore it has a 12 Attack Rating.
If we only have an NPC's THAC0, we just take the long way to get there like we learned last time: 12 + (19 - THAC0). A zombie has a THAC0 of 18, therefore it has a 13 Attack Rating.
And that's all the conversion done. Let's see how it all comes together.
Defense Rolls
So now PCs have a DEF bonus instead of AC, and NPCs have an Attack Rating instead of Attack Bonus, and that finally gives us: the Defense Roll.
The Defense Roll is another reactive d20 roll, like a saving throw, where PCs attempt to roll high to defend, dodge, or guard against attacks in combat that target them:
Defense Roll: 1D20 + DEF ≥ Attack Rating
If the PC's Defense Roll is equal to or greater than the NPC's Attack Rating, the PC successfully defends against the NPC's attack and avoids damage. If the PC's Defense Roll is less than the NPC's Attack Rating, the PC is hit by the attack and takes damage.
GM: Randy, roll to defend against the goblin's spear!
Randy: I rolled an 8... and with chain mail and a shield I have a +5 DEF - 13 total!
[The GM compares Randy's Defense Roll of 13 to the goblin's Attack Rating of 12]
GM: Randy, you pull your shield up and swat the goblin's spear away just before it pierces into you!
And there we go.
In two posts we've first updated Basic D&D to use Ascending Armor Class and Attack Bonuses, then converted those into PC Defense Rolls and NPC Attack Ratings - and we've preserved all the math along the way! It's all fully compatible, whether you're running the hot new OSR zine using OSE, or running old TSR modules with Rules Cyclopedia.
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