What level should a shorter-duration “Mass Protection from Energy” be?

Question

The group I DM for (7 characters - Bard, Cleric, Druid, Fighter, Ranger, Rogue, Wizard - all level 7) will soon go up against an Adult White Dragon. (They don't know yet. Heh.)

I would like to gently introduce them to the art of researching enemies and preparing for encounters, with the help of their conveniently-high-level spellcaster mentor (class left open for flexibility). In the present case, he would suggest Protection from Energy against cold damage.

Unfortunately, this 3rd-level abjuration only targets one creature - and it requires concentration. So our three characters capable of casting the spell (the Cleric, Druid and Wizard) could, between them, only protect three characters. Conversely, the duration of one hour is really far too much if they can choose the time and place of the battle.

So I am thinking of their mentor handing them (or their finding) a scroll of Mass Protection from Energy, which functions just like Protection from Energy, but protects up to ten creatures, with a duration of up to one minute (with concentration).

Question: what would be an appropriate spell level for Mass Protection from Energy?

Is there a mass version of Protection from Energy? is related, but for dnd-3.5.

Answer 1

I recommend a different approach: Potions of Resistance

Your current thinking requires developing a homebrew spell, which is likely in the ballpark of 6th level (see the Spell Level section at the end of my answer), and the scroll sets your NPC back 2 months, and 15,000 gp (using the Scribing a Spell Scroll rules in Xanathar's Guide to Everything).

While this is certainly approachable at higher tiers, level 7 characters are typically not privy to these kind of favors. Instead, potions of resistance seem much more in line with the types of things your party could be provided. They are Uncommon potions (instead of Very Rare spell scrolls), meaning your NPC only needs to expend one workweek and 100 gp for each potion (using the Crafting an Item rules in Xanathar's Guide to Everything).

Brewing 10 of these potions costs 15 times less than scribing the spell scroll, and the time investment can more easily be pushed off on less experienced casters. They come with the added benefit that a party spellcaster need not succeed on an ability check to cast the spell:

If the spell is on your class’s spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell’s level. On a failed check, the spell disappears from the scroll with no other effect.

...and they last an hour so the party can consume when they are near the dragon rather than having to wait until the dragon shows up to hurriedly cast the scroll.

Spell Level

If you do wish to stick with a scroll of mass protection from energy, the spell you describe is likely ~6th level. I evaluated this based on the difference between the spell heal and mass heal, and using the spell points rules in the Dungeon Master's Guide.

Mass heal provides 10-times the effect as heal, just like your homebrew spell would. Heal is a 6th level spell, which converts to 9 spell points, and mass heal is a 9th level spell, which converts to 13 spell points; an increase of 4 points.

With all of this in mind, we can take the base spell point cost of protection from energy, which is 5, and apply the change we saw with heal. This puts our mass protection from energy at 9 spell points, which is a 6th level spell.

Note: I use difference in spell points instead of just spell slot level (which would have the same result in this specific case) because the difference in power between spell levels is not necessarily linear, as willed out by the non-linear nature of the spell point table

Answer 2

Concentration is a powerful tool for balancing spells

Many spells have a rule for upcasting them to target more creatures. This includes spells that can heavily influence the results of combat, such as Banishment. Now of course, Banishment is somewhat different, because it can fail due to the saving throw. However, the influence of Protection from energy is arguably less pronounced unless the monsters faced deal only one type of elemental damage which is unlikely. It is also noteworthy that the spell does not protect at all against physical damage types. Anyway, requiring concentration is a lot more important for maintaining balance than restricting the number of targets to one. The concentration is also more important than the exact duration. If you consider only combats, the difference between a minute and an hour are mostly irrelevant. Both are usually for one combat.

Ten is a lot

Now, considering the usual spells that allow more targets on higher spell levels, ten targets is a lot. Most of these spells allow for one additional target per extra level. However, since we remove the flexibility (there is just one version and we have to expend a spell slot that high) we can be more liberal on the number. So rather three extra targets per extra level. For a level 6 spell.

Have your characters come prepared (Alternatives to creating a modified spell)

Note that if you just use a scroll, there is no need to determine a spell level. You just give your players an item that has the desired effect on narrative and gameplay. It does not even have to be a spell scroll. Also note that you say you want to have your players research their opposition. Protectiction from energy does not really do that. Given that you can chose a damage type when casting you can just take that spell into a dungeon, look at the boss and then protect people against the right damage type. So, having your players chose a scroll for a specific damage type might be more beneficial in this context. What you probably should do to implement research as important task is giving the opportunity to gain a specific benefit per adventure (as item, blessing, or otherwise) but make it so they have to chose one and to chose "correctly" they need to do research.

Answer 3

I suggest not making a homebrew spell.

Instead of having a spotlight-stealing mentor solving their problems for the player for free, instead make the PCs the real heroes of preparing their eventual dragon fight themselves and paying up for their entire preparation.

Having the Cleric and Druid prepare Prot vs Energy is a very simple thing.

The mentor could also teach the Protection from Energy Spell to the Wizard. Even better: Send the party on a mini-side quest for the Wizard to actually find the spell, and learn it, from some old book by an ancient wizard, whose wizard tower is now in ruins. Now guarded by magic puzzles and monsters, of course.

In addition to normal potion crafting cost and time, a special ingredient could be needed: each Potion of Protection from Cold needs the heart of a "powerful cold creature". So, the party goes on a short "single game session" side quest to learn that, in a high mountain pass not too far away, a pack of Winter Wolves is rumored to have terrorized that area for years, blocking the pass for use by travelers. The pack is led by an older, stronger, more cunning Winter Wolf pack leader. The party brings back enough hearts to craft the potions they need, plus a few extra.

During that little adventure they could meet with and help a "Blue Satyr", a cold-based satyr, that is willing to allow the Ranger to learn, temporarily, the Protection from Energy spell, usable once a day, by giving him a tiny magical blue sprite as a reward. "But the more you ask for its help, and the longer it has to stay with you, the more it will want to go home, and thus the harder it will be to convince it to remain with you." Basically, each time the Ranger uses the sprite (the Ranger must use his Action to use the spell-sprite), he must make a CHA Save with increasing difficulty, say initial base DC 8, +1 per previous casting, +1 per week elapsed from the beginning, until a failed the save, and then the sprite says goodbye and leaves. In other words: Ranger also gains the needed spell, but not in a long-term unbalancing "I have more known spells than I should have" fashion.

Transport to the side quest areas could also come at a cost, if they need special teleportation or other forms of fantastic transportation services.

So, players don't like to spend their gold, sure, but at level 7 they should have more than enough gold to pay for everything. If not, add another mini-adventure, this time to go get a pirate's gold chest or something with a not much magical items but a good "cash" payout. In any case, the dragon's hoard will surely more than compensate for any "war preparations" expenses needed (just adjust the final treasure).

Ding it like this has the following advantages:

• Extra game sessions, the preparation for killing the dragon is a small adventure in itself, or maybe even three.
• No need for Concentration checks when taking damage from the dragon. That kind of fight lasts long enough and you'd need repeated castings mid-fight to make the resistance last more than a couple rounds. Using potions it is a way more reliable form of protection.
• PCs become 100% proud of their potions.
• The mentor is a guide, not stealing the spotlight of the PCs' capabilities.

Heck, the mentor could even be a level 1 old wise man instead of an all powerful figure, and it would work. Personally as a player I really hate those vitally important DM-PCs around which the entire quest seems to rotate. It make me want to say : "Hey powerful mage why don't YOU go solve the problem if you think it is so important?". It makes more sense for the NPCs to help the PCs without overshadowing them.

If you feel that your NPC is too powerfully overshadowing the players, try this: Some strong enemy attacks the powerful NPC mentor, and there is a great magical battle. With help of PCs, the mentor wins, even then he has to use a WISH (maybe by using an item because while high level, he is not level 17+), and magic too powerful so his "soul" is badly wounded, and he forever more losses ALL of his magical powers. Cue dramatic scene. So you get a high HP, high knowledge mentor, but not an NPC that risks outshining the PCs. "Now my young friends the future of the world lays even more on YOUR shoulders!"