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I'm a D&D 5e system GM who runs a West Marches style campaign where I run a session a week with all sorts of adventures. This means I borrow a lot from the internet and quite often take monsters from other systems and convert them.

I generally have a good feel for what I want the encounter to be, and make any conversions generally match the difficulty I'm after. Sometimes however it might be nice to run monsters/encounters as they were originally designed (if this is even possible)

I have recently been looking at this Pathfinder asylum adventure which inludes the following "Madness Effect", summised:

While in a visual patch of madness all light begins to strobe at irregular intervals. This strobing also affects darkvision. When the strobing starts, each PC needs to make a DC 24 Will Save or suffer a −8 penalty to Perception checks, and a −4 penalty to all other actions. A successful save reduces this penalty to −4 and −2. Additionally, concentration checks must be made to cast spells successfully (DC 15 + spell level × 2). During combat, a DC 15 Perception check must be made to target an enemy, otherwise, the enemy appears to not be there when the attack is made, or if there is a friend within 5 feet of the intended target, that PC is targeted by the attack instead.

I would normally take the Will Save and penalties to Perception checks and just make the an appropriate scale in my campaign. These however seem really high, which makes me think that Pathfinder skills go up way higher than D&D 5e, either that or these are really difficult effects to save against, or maybe a combination of the two.

My question after the relevant rambling is: Is there any relationship between these Pathfinder difficulty classes and D&D 5e difficulty classes?

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  • \$\begingroup\$ In general, it is better to ask specific questions, and allow answerers to offer broader answers if they exist. That way you at least get what you need, and avoid having an overly-broad question where you risk the answer being “no, at least in general.” All the existing answers have tackled the specific example case, so I think this is a better way to phrase your question. Do feel free, though, to roll back if you feel this is not truly the question you wish to ask (but if you really do want to get the general answer, I think you need to focus more on that rather than a specific example). \$\endgroup\$ – KRyan Oct 26 '18 at 14:28
  • \$\begingroup\$ @KRyan that's really great advice, thanks. Both answers have been super helpful but are focused on different answers. You are completley right. \$\endgroup\$ – GPPK Oct 26 '18 at 15:00
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Pathfinder characters usually add a number equal to three plus their level to skills with which they are trained, or "proficient" in D&D 5e terms. A D&D 5e character gains a much smaller proficiency bonus that maxes out as +6 at 20th level.

When converting between the two, remember that D&D 5e made two major design evolutions over its predecessor, D&D 3.5, which Pathfinder is heavily based on:

  1. D&D 5e now uses bounded accuracy. In short, numbers are much smaller, and you'll almost never see something like a DC 24 Wisdom Save. The CR 24 ancient Red Dragon's breath weapon is DC 24.
  2. D&D 5e simplifies the bonuses and penalties of D&D 3.5/Pathfinder to a simple system of Advantage/Disadvantage. It was felt that the extra complexity of counting small numbers detracted from the gameplay more than it helped. You won't often see penalties to Perception checks, but instead Disadvantage on Perception checks.

As a result, you will never see anything as complex as the quoted text as an area effect in D&D 5th edition. How this might work in 5e is simply as follows:

While in the area of strobing light, all creatures have disadvantage on attack rolls and Wisdom (Perception) checks. Creatures which do not rely on sight are immune to this effect.

The D&D 5e Dungeon Master's Guide, p, 230 gives excellent guidelines for setting a DC for a saving throw or skill check. Ask yourself whether this check should be Easy (10), Moderate (15) or Hard (20).

The rules on Advantage/Disadvantage (DMG p.239) suggest that this situation is one where that applies (emphasis mine):

Consider imposing disadvantage when...

  • Circumstances hinder success in some way.
  • Some aspect of the environment makes success less likely (assuming that aspect doesn't already impose a penalty on the roll being made)

Note that imposing a skill penalty and allowing a saving throw against the effect are still valid mechanics in D&D 5e, but in general, these are much simplified to use the Advantage/Disadvantage system.

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Unlike 5e, D&D 3.5 (and, by extension, Pathfinder) doesn't include bounded accuracy in its design philosophy. As a result, high-level characters are able to accrue much higher bonuses to their various stats, including skill checks and saving throws. For example, most 5e characters will never see a skill check bonus higher than +11, while in Pathfinder it's possible to exceed that bonus even at level one, and high-level characters can easily attain skill bonuses of +30 or more. While the linked page doesn't seem to state what level of PC's it's intended for, it looks like it's built for a party of around level 8-10, perhaps slightly higher. Therefore, I'm going to evaluate these numbers in the context of what they'd mean to a level 10 Pathfinder party, and try to convert them into numbers that would have similar meaning to a level 10 5e party.

each PC needs to make a DC 24 Will Save

At this level, a DC 24 will save is still fairly difficult for many characters - a character specialized in the relevant stats (e.g. a cleric) would generally have a better than even chance of succeeding, but other characters would struggle. A typical fighter would likely fail the save around 75% of the time. Converting those probabilities into numbers for a different system is always somewhat inexact, but a DC 16 wisdom save would be a comparable 5e equivalent.

or suffer a −8 penalty to Perception checks, and a −4 penalty to all other actions

In 5e, it's atypical for circumstances to apply numerical bonuses or penalties. Instead, I'd suggest simplifying this mechanic to "disadvantage to all rolls on a failed save, no effect on a successful save".

Additionally, concentration checks must be made to cast spells successfully (DC 15 + spell level × 2)

In 5e, concentration checks are generally handled using constitution saves, but here it would likely be more appropriate to have them save using their primary spellcasting stat. In Pathfinder, the DC seen here is approximately a 50-60% chance of success, assuming the character is casting their highest-level spells. 5e doesn't generally scale DC's based on spell level, so it would be more appropriate to use a flat DC instead. Personally, I would use a DC of 16 here. Depending on how the numbers work out, it might have slightly higher odds of success than the Pathfinder version, but players no longer have the option to improve their odds by deliberately casting weaker spells, so I'd say it evens out. Plus, I'd like to keep the same DC across all saves against the effect, and this matches the DC used for the earlier wisdom save.

During combat, a DC 15 Perception check must be made to target an enemy, otherwise, the enemy appears to not be there when the attack is made, or if there is a friend within 5 feet of the intended target, that PC is targeted by the attack instead.

For a level 10 Pathfinder character, DC 15 is pretty trivial. When not impaired, most characters will auto-pass this check, or at least have very low odds of failure. However, since we're using disadvantage instead of a flat numerical penalty, we can't drive the DC too low. Assuming most of your characters are proficient in perception, DC 8 would be reasonable. If most are nonproficient, DC 6-7 might be more appropriate.

So, bringing it all together:

While in a visual patch of madness all light begins to strobe at irregular intervals. This strobing also affects darkvision. When the strobing starts, each PC needs to make a DC 16 Wisdom Save or suffer disadvantage on all d20 rolls. Additionally, casting a spell successfully requires a DC 16 save using the spellcaster's primary casting stat. During combat, a DC 8 Wisdom (Perception) check must be made to target an enemy, otherwise, the enemy appears to not be there when the attack is made, or if there is a friend within 5 feet of the intended target, that PC is targeted by the attack instead.

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Scaling the difficulty class of skill checks between D&D 5e and Pathfinder

(This answer is based on the 5th printing of the Pathfinder Core Rulebook in November, 2011, the first printing of the D&D Player's Handbook in August, 2014, and the first printing of the Dungeon Master's Guide in December, 2014.)

D&D 5e has a much narrower range of character competency than Pathfinder by way of the game designers' "bounded accuracy" approach to action resolution. Characters' level-based proficiency bonus ranges from +2 (from levels 1 through 4) to a maximum of +6 achieved at level 17. If a character is proficient with the type of check in question, the proficiency bonus applies, if not, it does not. There are some special cases where class features can increase this as well.

Pathfinder, on the other hand, uses skill ranks for a bonus. If a character hasn't taken any ranks in the skill in question there is no bonus. Otherwise, the character's bonus is the number of ranks taken + 3. Characters may not take more skill ranks in a skill than they have hit dice (levels). This means a proficient character's bonus ranges from +4 at first level to +23 at 20th level for a skill in which she or he is proficient. There is the complicating factor that a character could forgo deepening development in a particular skill in order to increase breadth of proficiencies, which I'm ignoring for the sake of this answer.

Ability score bonuses and penalties are the same in both games, so for simplicity this discussion will assume average ability scores. Likewise, characters without proficiency in a skill receive no bonus and these too are excluded from the discussion.

One might consider a simple arithmetic scaling where the range difference is normalized and the base adjusted such that one subtracts 2 for the base difference then scales the increase by 19:4. This would make a difficulty class (DC) 15 in Pathfinder into a DC of 2 + ((15 - 4) * 4 / 19), or about 4 in 5e. This does not, however result in similar chances for success for a proficient character except at a specific level and so is not going to meet your goal of running the encounter as originally designed.

The fundamental problem is in how the games approach DC. In text that is apparently no longer available at the Wizards of the Coast web site, Rodney Thompson, a game designer who has worked on a number of d20-based role-playing games with Wizards of the Coast and other publishers, wrote this regarding bounded accuracy and DC:

Thus, our expected DCs do not scale automatically with level, and instead a DC is left to represent the fixed value of the difficulty of some task, not the difficulty of the task relative to level.

And there's the rub. In Pathfinder, a first level proficient character with an average relevant ability score has a 45% chance to succeed at a DC 15 task. That Pathfinder character's chances of success at that task, and any in which he or she is proficient, improve by 5% per level.

In order to play a Pathfinder encounter as designed applying D&D 5e rules, you'll need to account for the level of characters the encounter was designed for. From there you can figure the designed chance of success and recreate that chance in your 5th edition game. For example, a DC 15 check is a 45% chance of success for a proficient 1st level Pathfinder character. This equates to a DC 13 in 5e. A DC 25 check designed for a 10th level Pathfinder character equates to a 40% chance of success, or a DC 16 in 5e.

Both checks in this example fall into the "moderate" category per page 238 of the 5e DMG, with the second example being tweaked slightly higher for upper tier play.

TL;DR Use the intended level of the Pathfinder encounter to calculate absolute difficulty for 5e and consider adding 1 or 2 to the DC to tweak it for the top tier levels of play.

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