If 6th+ level spells are making high level play tricky for you as a DM, replace a 6th level slot with a 5th and 1st, 7th with 5th and 2nd, 8th with 5th and 3rd, and a 9th with a 5th and 4th. Same net power, lower power ceiling.— Mike Mearls (@mikemearls) December 19, 2018
This houserule is built on the premise that all spell levels are equal, so that two 1st-level spells are worth the same and have the same sort of output as one 2nd-level spell, three 1st-level spells matches a single 3rd-level spell, and so on and so forth. This is largely consistent with the rules as presented in the Dungeons & Dragons Fifth Edition core books: we can refer to either the "at higher levels" entry of spells in the Player's Handbook or the Spell Damage table under Creating a Spell in the Dungeon Master's Guide to confirm that, as a rule, each spell level adds one die of damage. Technically then, the house rule above ensures that spellcasters maintain their existing potential for damage-dealing/healing.
Of course, things aren't as simple as that in practice for multiple reasons. Official spells play fast and loose with this formula and certain legacy spells break it completely. Your sorcerer will never have access to the insane meteor swarm. Then there are spells that don't deal damage, but introduce world-bending effects (like Wish, say). Additionally, a lower cap on spell level restricts the amount of damage spellcasters can deal during a single turn. The caster might be able to do the same damage in the long run, but it'll take them longer and that makes things more dangerous for them and their party. If you implement the rule, be very wary of high CR monsters with large hit point pools.
On the other hand, it may have some positive side effects. For instance a healer has a lot more slots for cure wounds and other restorative spells, so that additional danger may be balanced out, in part, by an increased resilience.
In short, the houserule is technically fair but may have the consequence of making parts of the game a little harder, particularly combats against higher CR monsters.
Note that spellcasters are still extraordinarily powerful in the fiction, even if the effects they can manifest have become more limited. A 20th-level wizard may not be able to cast wish, but they can cast nine cones of cold which would be enough to single-handedly decimate a small army. We tend to use the terms "low magic" and "high magic", but this houserule might suit a setting in which magic is more "medium magic", falling somewhere in the middle.
I wanted to make Mike's houserule easy to visualise and implement so I went ahead and made a new Spell Slots per Level table based on it. Use this table in place of the spell slots section of the full caster class tables.
CLASS LEVEL
|
1st
|
2nd
|
3rd
|
4th
|
5th
|
|---|---|---|---|---|---|
| 1st | 2 | — | — | — | — |
| 2nd | 3 | — | — | — | — |
| 3rd | 4 | 2 | — | — | — |
| 4th | 4 | 3 | — | — | — |
| 5th | 4 | 3 | 2 | — | — |
| 6th | 4 | 3 | 3 | — | — |
| 7th | 4 | 3 | 3 | 1 | — |
| 8th | 4 | 3 | 3 | 2 | — |
| 9th | 4 | 3 | 3 | 3 | 1 |
| 10th | 4 | 3 | 3 | 3 | 2 |
| 11th | 5 | 3 | 3 | 3 | 3 |
| 12th | 5 | 3 | 3 | 3 | 3 |
| 13th | 5 | 4 | 3 | 3 | 4 |
| 14th | 5 | 4 | 3 | 3 | 4 |
| 15th | 5 | 4 | 4 | 3 | 5 |
| 16th | 5 | 4 | 4 | 3 | 5 |
| 17th | 5 | 4 | 4 | 4 | 6 |
| 18th | 5 | 4 | 4 | 4 | 7 |
| 19th | 6 | 4 | 4 | 4 | 8 |
| 20th | 6 | 5 | 4 | 4 | 9 |
One quirk of the house rule which is made very obvious when it is presented in table form: a rapid increase in 5th level spell slots, far outstripping growth of lower level slots. There are good reasons for this, but it makes for an odd evolution in the character's power which breaks with the game's norms. We expect higher spell levels to have equal or fewer slots to the spell levels that precede them.
Just for fun, I tried to make an alternative table which would convert the 6th-9th level slots down in the same way, but which would ensure that no spell level ever gets more slots than a lower spell level. This proved possible, but the solution to make it worked was a great deal more complex than Mike's "one 5th-level slot plus one [X-5]th level slot, where X is the original spell slot level" house rule. There is no universal formula here, the precise conversions had to be massaged to fit the rules I'd imposed.
The curious can read about these substitutions in detail immediately below. The rest should just skip ahead to the table!
- Class Level 11: The 6th-level slot is replaced by 1st-, 2nd- and 3rd-level slots.
- Class Level 13: The 7th-level slot at class level 13 becomes a 2nd- and a 5th-level slot.
- Class Level 15: The 8th-level slot is replaced by 1st-, 3rd-, and 4th-level slot.
- Class Level 17: The 9th-level slot normally granted at class level 17 becomes a 2nd-, 3rd-, and 4th-level slot.
- Class Level 19: The 6th-level slot is replaced by 1st-, 2nd- and 3rd-level slots.
- Class Level 20: The 7th-level slot at class level 20 is converted into a 1st-, 2nd-, and 4th-level slot.
CLASS LEVEL
|
1st
|
2nd
|
3rd
|
4th
|
5th
|
|---|---|---|---|---|---|
| 1st | 2 | — | — | — | — |
| 2nd | 3 | — | — | — | — |
| 3rd | 4 | 2 | — | — | — |
| 4th | 4 | 3 | — | — | — |
| 5th | 4 | 3 | 2 | — | — |
| 6th | 4 | 3 | 3 | — | — |
| 7th | 4 | 3 | 3 | 1 | — |
| 8th | 4 | 3 | 3 | 2 | — |
| 9th | 4 | 3 | 3 | 3 | 1 |
| 10th | 4 | 3 | 3 | 3 | 2 |
| 11th | 5 | 4 | 4 | 3 | 2 |
| 12th | 5 | 4 | 4 | 3 | 2 |
| 13th | 5 | 5 | 4 | 3 | 3 |
| 14th | 5 | 5 | 4 | 3 | 3 |
| 15th | 6 | 5 | 5 | 4 | 3 |
| 16th | 6 | 5 | 5 | 4 | 3 |
| 17th | 6 | 6 | 6 | 5 | 3 |
| 18th | 6 | 6 | 6 | 5 | 4 |
| 19th | 7 | 7 | 7 | 5 | 4 |
| 20th | 8 | 8 | 7 | 6 | 4 |
I'm pretty happy with this alternative. It preserves the expectation that the lower the spell level, the more slots the character should have. The difference in the number of slots also isn't too overwhelming (at 20th level, a character would have 33 slots compared to 28 with Mike Mearls' original house rule).
Since the spell slots skew to lower levels the same consequences discussed above naturally apply to this version of the house rule as well. But here they apply to an even greater extent. Therefore, this alternative table is best suited to deadly, gritty, low magic games, such as those with themes of survival and horror that more powerful magic would likely undermine.
