Dungeon Master Assistance

A place to share thoughts and ideas about Dungeons and Dragons

Threat creation using bounded accuracy

TrapThis is an expansion of an idea presented by the “Thinkerer GM” here: http://thinkerergm.wikispaces.com/Threat+creation+using+bounded+accuracy

He says: “This system allows to create a threat (monster, trap, etc) by extending the Difficulty Classes to apply it to other characteristics. ”

You can use these guidelines for monsters and traps that you can’t find in official sources, such as monsters that you are converting from an older module, or one of your own design.

Here is his table

Extended Difficulty Classes Table

Difficulty\Characteristic AC Attack
Bonus
HP Damage Save DC CR
Very Easy (DC 5) 8 +2 5 2 (1d4) 11 0
Easy (DC 10) 10 +3 10 4 (1d6) 12 1/4
Moderate (DC 15) 12 +4 20 8 (2d6+1) 13 1
Hard (DC 20) 14 +5 40 16 (2d10+5) 14 3
Very Hard (DC 25) 16 +6 80 32 (2d10+21) 15 5
Formidable (DC 30) 18 +7 160 64 (2d10+53) 16 7
Nearly Impossible (DC 35) 20 +8 320 128 (2d6+117) 17 9

 

This is the idea for Traps:
How hard is it to notice? [DC for WIS (Perception) check]
How hard is it to locate? [DC for INT (Investigation) check]
How hard is it to avoid? (If it is activated.) [Attack bonus]
How hard is it to deactivate? [DC for DEX (Disable device) check]
How hard is it to destroy? [Hit Points]
How hard is it to hit? (in the proper location or with enough force to cause it damage) [AC]
How hard is it to save against? (for poison and the like) [Save DC]
How hard is it to endure? [Damage]

Answer each of these questions with “very easy”, “easy”, “moderate”, “hard”, “very hard”, “formidable”, or “nearly impossible”, and look up the difficulty class table above.

Add all of the DC’s together and divide by 8 to get the average DC for the trap. Use this DC for the traps Challenge Rating (CR).

The idea is very similar for monsters:

How hard is it to detect? [DC for WIS (Perception) check]
How hard is it to avoid? [Attack bonus]
How hard is it to hit? [AC]
How hard is it to destroy? [Hit Points]
How hard is it to save against? (for poison or spells) [Save DC]
How hard is it to endure? [Damage]

Add all of the DC’s together and divide by 6 to get the average DC for the monster. Use this DC for determining the CR if the monster is encountered singly. Use the next higher DC for an encounter with two of these monsters. Increase the DC by one additional level for each doubling of the number of monsters in the encounter. For example, a DC 10 monster becomes DC 15 it there are two of them, DC 20 for 4, DC 25 for 8, etc.

For DCs above 35; For each additional level, increase the DC by 5 and increase the CR by 2.

When determining how hard it is to do these various things, you should be thinking about a single, 0 level commoner. This is what bounded accuracy is all about. Each activity is hard or easy regardless of the level of the characters attempting it. To determine how difficult an encounter this will be for your group use the CR and adjust for the number of PCs.

For Experience Points, you can use the XP values of similar CR monsters as a guide.

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