In 5th Edition D&D, there's a few circumstances where a character's damage might be "maximized".
For example, the Evocation Wizard's Overchannel ability:
Starting at 14th level, you can increase the power of your simpler spells. When you cast a wizard spell of 1st through 5th-level that deals damage, you can deal maximum damage with that spell.
—Overchannel, Player's Handbook, pg. 118
Or an entry on the Wild Surge table:
33-34 Maximize the damage of the next damaging spell you cast within the next minute.
—Wild Magic Surge, Player's Handbook, pg. 104
The way I see this, there's two valid ways to treat this effect:
- Treat the damage dice as though each die rolled its respective maximum value
- Treat the damage as though it is the sum of the maximum values that each possible die could have rolled
These two effects might seem similar, and in most situations they are, but there's a few circumstances where they might be different. For example, for an Attack-Roll based spell, the damage of interpretation 1 is doubled on a crit, because you're doubling the quantity of dice that are being used to calculate damage; but under interpretation 2, it would not be, because critical hits do not double flat damage modifiers, and taking the maximum value of all the rolled dice would turn it into a flat modifier.
Conversely, there are spells which depend on a specific value rolled on the damage dice to change its behavior, like with Chaos Bolt:
You hurl an undulating, warbling mass of chaotic energy at one creature in range. Make a ranged spell attack against the target. On a hit, the target takes 2d8 + 1d6 damage. Choose one of the d8s. The number rolled on that die determines the attack's damage type, as shown below.
If you roll the same number on both d8s, the chaotic energy leaps from the target to a different creature of your choice within 30 feet of it. Make a new attack roll against the new target, and make a new damage roll, which could cause the chaotic energy to leap again.
—Chaos Bolt, Xanathar's Guide to Everything, pg. 151
Under interpretation 1, the Chaos Bolt always deals Thunder damage, and always leaps to a new target on a successful hit, because each of the d8s are being treated as having each rolled 8. Under interpretation 2, however, the d8s are rolled, and then ignored for the purpose of calculating the total damage, because the damage is simply being set to the maximum possible value of 22, without setting the values of the individual dice.
So which is it? Is there rules support to show that Maximizing Damage should be handled one way or the other?
Additionally, since I've raised the spectre of an attack-roll based spell like Chaos Bolt, the issue of the Attack Roll itself also needs to be raised: should the Attack Roll be treated as an Automatic hit (or crit!) because failing to do so will result in the spell dealing less than maximum damage? Or is "Maximizing" damage meant to only mean damage after a successful attack roll, which therefore means the attack roll cannot be overridden? For spells that have Saving Throws, should the targets of these spells be treated as automatically failing their Saving Throws, since failing to do so would result in the spell not dealing maximum damage?