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In the AD&D 2e supplement, In The Cage: A Guide to Sigil, there are rules given for the impacts of a noxious smog, as follows:

Any PC exposed to [the smog] must make a Constitution ability check, with the usual bonuses for poison saving throws for dwarves and halflings.

The paragraph goes on to explain the effects of a failure, which are easy enough to transfer into 5e, so they aren't relevant here.

Based on the question Determining how to apply Bonuses and Penalties to Ability Checks, my understanding is that the DC of this check would be equal to the overall relevant ability score, which is Constitution in this case. (Please correct me if I'm wrong here; I've only ever played 5E.)

Is there a way to set the DC on this to appropriately difficult per 5E rules, or is this a case where the games are so dramatically mechanically different that it basically comes down to DM judgement? I am not asking for input on what DC everyone thinks this effect might have; I am asking if there is a tidy way to port this mechanic.

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Your best bet is to rebuild the hazard from scratch using the 5e rules.

Wizards, when they first published 5e, also released a handy Conversion Guide, with details about converting characters, monsters, magic gear, and even entire adventures. There are two types of conversions listed, Careful and Quick, and both suggest utilizing the new Trap rules from the Dungeon Masters Guide to rebuild the trap from scratch, with a DC and damage based on the levels of the characters, and the severity of the threat it's intended to place.

What we know about this smog is that it's A) it requires a Constitution save, which can be directly ported over, and B) it's a poison, so anyone with resistance/immunity to poison can apply that to their save. The save DC and amount of damage will be entirely dependent on the level of players expected to encounter the smog, and how dangerous the smog should be to those players.

As an additional resource, Xanathar's Guide to Everything (which was released long after this conversion guide was published) has additional notes for creating level-appropriate traps for your players.

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    \$\begingroup\$ Gotcha, I figured that would be the answer! This is a permanent environmental hazard (literally just a badly polluted part of a city), so I probably won't go with the guidance on traps, but I can use the general guides in 5e on how hard saves should be to figure it out. \$\endgroup\$ – L Cooper Jun 2 at 20:20
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    \$\begingroup\$ I think the guidance on traps is supposed to apply to general hazards as well, I don't know if there's separate guidance for those kinds of issues, but if that doesn't suit you, then the general DC guidelines should be sufficient I would think. \$\endgroup\$ – RevanantBacon Jun 2 at 20:25
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    \$\begingroup\$ My only concern about using guidance for traps is that this is a pretty large area, and I don't want to complete disincentivise players from exploring it; I see it as being more akin to bundling up to avoid freezing in a cold area than trying to dodge damage. (The effect of the smog is just a flat penalty to strength and constitution, with rolls repeating on strenuous activity. Basically just asthma!) \$\endgroup\$ – L Cooper Jun 2 at 20:29
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    \$\begingroup\$ The guidance in XGtE will probably be more appropriate then, as it has guidance on more nuanced traps than "Save vs falling into the spiky pit.", like traps that only cause inconveniences to the target, such as being snared or dropped into a non-spiky pit, or being put to sleep. As a side note, a flat penalty to Str and Con I think would likely translate into just giving them disadvantage on rolls made using those stats. \$\endgroup\$ – RevanantBacon Jun 2 at 20:33
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    \$\begingroup\$ Cool, I've actually got that book at home, but had mostly used it for the fun stuff that it adds for players-- I'll have to crack it open! I'll leave this open for the courtesy 24 hours, in case someone has some left-field brilliance, but you've pretty solidly solved this one! \$\endgroup\$ – L Cooper Jun 2 at 20:35
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In AD&D you would be rolling equal to or under the ability score. This gives you roughly half a chance of success when your ability score is 10 or 11. If you want roughly half a chance of success for an ability score of 10 or 11 in D&D 5, you set the DC to be equal to 10.

Note that this scales differently for very high and very low constitution scores, but there is absolutely no reason to assume anyone took this into account when designing the adventure, so don't worry.

Also note that both D&D 5 and AD&D have ability score inflation, so maybe the calibration should be done with an ability score of 12 instead, and since in D&D 5 this should be a constitution saving throw, some characters will have a proficiency bonus, too. If you want to be fancy and take this stuff into account, the AD&D character will succeed on 12/20 and we will want the D&D 5 character to succeed on 9+, so giving them a bonus of +2 (+1 for con +1 for proficiency as some kind of average estimate of how much it matters) leads to DC 11. Which is essentially the same as above.

So the DC should clearly be 10 or a bit more. If the adventure is high level, maybe use 12 instead. This would lead to roughly as many failures as the original.

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