How do I recalibrate this encounter from D&D 3.5e's Red Hand of Doom for D&D 5e?

Question

I'm running D&D 3.5e's Red Hand of Doom updated for 5e. So far it has been effortlessly straightforward. I've been keeping the basic combat encounter structures and simply swapping out 3.5e monsters for similarly-named 5e monsters (3.5e hobgoblins for 5e hobgoblins, 3.5e manticores for 5e manticores, etc.). Likewise, I've been winging ability checks and other non-combat challenges just by eyeballing how hard the stated 3.5e DCs likely would've been and using my best judgment to apply 5e DCs of roughly similar probability given bounded accuracy.

However, the PCs are coming up on a critical encounter with a major non-combat objective that presents special conversion problems. The encounter in question, arising near the end of Part I, is

Skull Gorge Bridge, where the PCs are tasked with destroying the eponymous bridge while facing heavy resistance.

Because this objective isn't a monster, I can't simply turn to stock 5e monsters and assume all the calibration will have been done for me. At the same time, it's not as simple as an ability check that I can just eyeball. RHoD provides 3.5e combat statistics for the objective (see p. 34-35), but I'm not sure how those statistics translate to 5e. As written, the objective has what I perceive to be an outsize pile of HP, plus additional defensive features (taking reduced damage from certain sources, etc.) arising from how 3.5e treated entities in the nature of this objective. It's not clear whether, or how, 5e might expect me to recalibrate those statistics and features.

That is problem enough, but RHoD also goes out of its way to enumerate 3.5e spells that can interact effectively with the objective -- most of which are either unavailable or fundamentally changed in 5e. To wit:

• Soften earth and stone does not exist in 5e.
• 5e's version of stone shape restricts the affected area to "no more than 5 feet in any direction," which was not a limitation of the 3.5e version.
• Stone to flesh does not exist in 5e.
• Transmute rock does exist in 5e and is substantially more useful in that it applies to any nonmagical rock, rather than only natural, unworked rock as did the 3.5e version. The 5e version could probably deal with the objective in a single turn, whereas RHoD says the 3.5e version just dealt some modest damage if used in a particular way.

Given the different combat mechanics and spell functions between 3.5e and 5e, how do I convert this encounter so it remains an appropriate challenge?

(In case context is helpful, the party is level 5 and comprises a Light cleric, a Hunter ranger, a melee-heavy Battlemaster fighter, and an Abjuration wizard. Despite RHoD being written for 3.5e parties starting at level 6, up to this point these 5th-level PCs have been able to handle the encounters in Part I of the adventure.)

Answer 1

Seeing as there have been no other answers, I will provide an answer from my own experience actually running this encounter.

To preserve the spirit of the original 3.5e encounter under 5e rules, the challenge of the non-combat objective needs to be reduced if the PCs are below 9th level.

The original encounter presents options -- not many, and not easy, but clearly more than just a single railroaded approach. The PCs can take out the defenders first and then turn attention to the main objective, of course. Or they can try to sabotage the objective first, probably with magic, which is presented as the most effective means of affecting it directly.

However, many of the sabotage tools a party would've had in 3.5e are simply not available, or are available only at higher levels. As written for 3.5e, RHoD names two spells available to PCs of 5th level or lower that are effective for this sabotage task. Soften earth and stone, available to 3.5e PCs of 3rd level, can theoretically do the job in one casting:

The weak spot under the southeast tower needs only 40 damage to collapse the bridge, whereas each other 5-foot section of the bridge has a whopping 270 HP. Soften earth and stone, if used on the ground under the bridge, deals 1d4x10 damage to the bridge section above it. RHoD, p. 33. If used on the ground under the weak spot, this spell has a 25% chance of dealing 40 damage and collapsing the bridge in one turn.

But as mentioned in the question, soften earth and stone doesn't exist in 5e. Likewise, stone shape, available to 3.5e PCs of 5th level, is explicitly described as being able to do the job in one "clever" casting, RHoD, p. 33 -- but that "clever" casting depends on the spell's area being larger in 3.5e than it is in 5e. Besides, in 5e, stone shape is a 4th-level spell only available to PCs of 7th level or higher.

As a result, using 5e rules, PCs heading into this encounter would need to be significantly higher-level to have tools of equivalent "one shot" efficacy. PCs in 5e get access to transmute rock at 9th level, which -- as the questions states -- can easily do the job in one turn. No lower-level 5e spell, as written, will do so. Worse, RHoD encourages DMs to limit tools the PCs do have, saying 1st- and 2nd-level spells in general "won't do much to the [objective]." RHoD, p. 33. Taking that instruction literally, a party of 5e PCs under 9th level has even fewer spell options that might be effective.

Granted, spell scrolls in 5e can give PCs access to spells above their level -- but they are not guaranteed to work. Using a scroll to cast a spell of a higher level than the caster could normally cast requires an ability check, and failure means the scroll is wasted. That is dangerous in itself, but more so given that 5e assumes a smaller distribution of magic items than 3.5e did overall, at least in my many years of experience playing both systems.

Moreover, casters in 3.5e typically could cast more spells in a day thanks to bonuses from ability scores, but those bonuses no longer exist in 5e. So even if a 3.5e party only had lower-level, less-effective spells available, they probably could cast them a greater number of times in the encounter before exhausting their resources.

When my 5th-level PCs faced this encounter, the only spell they had available that even came close to the parameters for efficacy originally defined in the adventure was shatter -- and only if upcast with a 3rd-level spell slot, of which the party's wizard had a total of two. As written, an upcast shatter would've been incapable of dealing the damage necessary to do the job in one casting, and meanwhile it was competing with 3rd-level spells that could've provided serious support (e.g., hypnotic pattern) or utility (e.g., fly) in the sabotage effort. In short, the party really had no option other than to fight. Nothing they had access to would've given them the chance a 3.5e party of equivalent level would've had to hit the objective hard in one turn and get out.

Had I to run the encounter again for a party of similar level, I would

reduce the HP of the normal bridge sections by 50% to 135 HP, and rule that the bridge's weak spot has vulnerability to thunder damage such as that dealt by spells like shatter.

These adjustments would give a 5e party more options, while still providing substantial challenge. After all, the PCs would still have to

find the weak spot before they can target it,

and the defenders surely won't make that easy.

Answer 2

When I ran this I made only two changes.

First I said a person with a pickaxe or proficiency with masonry tools (and the tools) could do their strength bonus X D20 damage (so a player with a strength of 18 (+4) could do 4d20 damager per round) to a section of the bridge as an action. Not that unreasonable, since a huge guy taking a tool for breaking stone to a stone bridge can do a lot of damage.

Second I said the mold earth cantrip functioned like the soften earth and stone for the purposes of bringing the bridge down by undermining it. I just said the age of the bridge combined with the weathering of the gorge made it uniquely vulnerable. That prevented it from being a major change to the function of the spell. I let my players roll an intelligence check or just gave the knowledge to anyone with proficiency with masonry tools that asked about the bridge.

It is worth noting I had 7 players for the first half of RHOD, so I was usually trying to make encounters harder not easier. Although in this encounter I did that my changing the stats on the dragon and playing him like a @^&@# who kept using flyby and trying to shove players off the bridge.