House Rule - Upcasting Enlarge / Reduce to extend duration

Question

The party is deep into my 5e-updated classic Greyhawk Giants series.

The hill giants and their orc and goblin minions have attacked the Sterish city of Headwater and have taken about a quarter of the city. The party is about to embark on a mission to go behind enemy lines, kidnap and assassinate a stone giant who is critical to the hill giants' city / siege offensive as being their only trained engineer. While the party's patron recognizes that the stone giant needs to die, she also recognizes that at present the Stone Giant Thane has not joined the giant alliance and she wants to keep it that way. Thus, she is requesting that the party carry the body of the slain giant honorably back to his Thane in an effort to preserve a fragile peace between the humans and the stone giants. I would like the party to be able to accept this mission, without it becoming either a logistical challenge involving wagons and draft animals, or without loaning them a portable hole.

Instead, I would like to provide them with a version of the Enlarge / Reduce spell which is in all aspects identical to the original except that it can be upcast to extend the duration.

I figure the giant is 17 feet high and 1000 pounds; after reduce it would be 8.5 feet and a manageable 125 pounds.

For this version of Enlarge / Reduce I am proposing that:

• When cast at 3rd level against living creatures, it lasts 10 minutes

• When cast at 3rd level against objects, it lasts 1 hour

• When cast at 4th level against living creatures, it lasts 1 hour (similar to polymorph)

• When cast at 4th level against objects, it lasts 8 hours

• When cast at 5th level against living creatures, it lasts 8 hours (much less than geas)

• When cast at 5th level against objects, it lasts 24 hours

The party Wizard is currently 8th level and getting close to 9th. Requiring her to use both her fourth level slots and maintain concentration for the duration of travel every day in order to move swiftly and stealthily into the mountains with the body is just the right level of challenge for the group.

My only concern is that allowing this version of the spell to the party wizard permanently will have some unforeseen interaction that I will later regret with some other spell, ability, or item. This question, for example, asks about upcasting enlarge to permit two changes in creature size, and the answers identify the interaction with levitation and grappling being problematic. I am interested in a similar troubleshooting review.

Answer 1

Tl;dr: This is doable. However, for this situation, I would give the party a single-use glass bottle that sucks an object inside; upon breaking the bottle, the item returns to normal. ie a ship in a bottle. If desired, the bottle can be fragile, so the party has to be careful with it while hauling the giant to the thane. Below are my (long) thoughts on altering spells.

When a spell is homebrewed to enhance its effects, such as allowing an upcast of Grease to last longer or Immolation do more damage, it changes a few things.

1. Resource Management. If the player uses fewer spell slots, what can they do with the extra slots?
2. Action Economy. If the player is doing the same thing in fewer turns, what are they using the extra turns for?
3. Versatility. If the upcast spell duplicates the effect of another spell, why would they bother learning that other spell?
4. Stacking. Does the change allow a character to stack effects or actions they otherwise could not?

In the case of allowing Enlarge/Reduce to bump multiple size categories, the spell becomes much more versatile. It can situationally replace Fly, as a character can suddenly reach (and jump) much higher locations. It can replace Hold Monster, since a character can now effectively grapple much larger creatures, or more easily overpower smaller ones, while also changing the check from Hold Monster's Wisdom save to a grapple check. And so on.

This makes it hard to anticipate the results of the change. Let's look at the above considerations in conjunction with extending Enlarge/Reduce's duration.

1. Resource Management. As noted in raithyn's answer, the way spell slots are used will be different. The player must sacrifice higher-level spell slots instead of multiple low-level slots; what can they do with the low-level slots, and what could they have done with the higher level slots? Generally speaking, a higher level slot is worth more than its "number" in lower spells; for example, a Fireball at 3rd level does more damage than a level 1 spell and a level 2 spell combined. For utility spells, this is messier to consider.
2. Action Economy. The spell originally lasted long enough for an encounter; by allowing it to last longer, the action economy is almost unaffected, since out of combat action economy is rarely relevant. It would impact a long encounter, such as an hour-long siege, but generally speaking, such encounters are about wearing the characters down, not split-second danger, and as such the resource management is of greater concern than action economy for that instance.
3. Versatility. Does extending the duration of Enlarge/Reduce allow it to replace other spells? Not likely. Maybe Polymorph, which can also alter the size of a creature. Since Polymorph's use case is rarely about just size, I would be comfortable allowing that possibility to exist.
4. Stacking. This is the only potential issue I see with the proposed change. By allowing this, a character could be Enlarged for an entire dungeon dive (as an example), effectively allowing the party to stack Enlarge with another action, exploration. For example, this means when the party is ambushed, the tank is already big. In this case, the spell requires concentration, so obtaining these results still comes at a cost; if the spell did not require concentration (such as Fire Shield), it would enable stacking a lot more effects/actions and thus be more dangerous to allow.

In the balance of all, we have one other variable that is harder to define. Player Choice. How likely are your players to abuse the change in some way? This can only be answered by you.

If it were my table, and I couldn't do the glass bottle thing mentioned above, I would allow the altered spell and deal with any issues if they came up, as I don't see this change causing many problems.

Answer 2

Possibly. It depends on your table.

Enlarge/reduce is designed to last through a single fight. By design, your wizard should expend a new spell slot during each encounter to maintain the effect throughout each.

From there, the question is, what does your adventuring day look like? If you were to use 6 encounters per day, the wizard should burn all of their 2nd and 3rd level spells to maintain the effect through a 16 hour period. This totals to 15 spell slot levels.

Do you allow your wizard to gain the benefits of a short rest while concentrating on a spell? RAW, you probably should but many groups I've played with don't. If not, then your 8th-/9th-level wizard loses 4 spell slot levels to opportunity cost (Arcane Recovery). Even with this, your wizard is losing much less by using a 4th-level or 5th-level slot to move the object all day (8 and 9 spell slot levels, respectively).

How balanced this is depends on how often your wizard is making concentration checks and, to a large degree, how much their standard play style depends on concentration to begin with.

• Signal Concentration Checks: If enemy NPCs always attack the party meaty shield, then the wizard has no reason to worry about concentration checks and it makes sense to enlarge that character for as long as possible. It will feel off to your players if your NPCs are suddenly attacking the wizard without warning. I'd suggest giving a visual cue to the spell that the wizard is providing the effect. This will tell your players that if the fighter/paladin is enlarged, the wizard will need to play defensively.
• Playstyle: If your wizard spends all their slots on fireball, upcasting enlarge/reduce takes away one use of that per day (which actually gives some back if they would cast the original enlarge/reduce every encounter), but otherwise has little effect on their resources. If they favor ritual spells (concentration required for any casting over 1 action), utility spells like detect magic, buffs like magic weapon, or battlefield control like hold person, committing to concentration for hours at a time is a major decision--one your wizard is likely to not select unless they must.

Whether these elements add up to balance depends on the conventions around your table. My spellcaster rarely uses concentration spells and my barbarian is all about grappling. The knock-on effects of allowing this buff would quickly outweigh the player resource cost. I might allow the upcasting to only affect objects, or provide a magic item that facilitates the upcast with the understanding that it is a loaner and the party will face consequences if they run off with it.

If, however, your wizard embraces utility and regularly has to decide between maintaining concentration on their current spell or switching to a new effect, this upgrade will likely not significantly change the balance of your campaign.