At 10th Level (Master Thief), thieves are able to decipher magical writings and utilize scrolls of all sorts, excluding those of clerical, but not druidic, nature. However, the fact that thieves do not fully comprehend magic means that there is a 25% chance that writings will be misunderstood. Furthermore, magic spells from scrolls can be mispronounced when uttered, so that there is an increasing chance per level of the spell that it will be the reverse of its intent (PHB, p. 27)
I’ve got two thieves approaching level 10—it’s time to refresh myself on thieves casting from scrolls. To translate the above, 10th+ level thieves can read magic user, illusionist, and druidical spells, in addition to the Protection scrolls usable by any class (DMG, p. 128: “Protection scrolls can be read by any class or race of character even without a magic spell”). While Intelligence can limit spell casting, “Those characters able to read and employ scroll spells may do so regardless of other restrictions” (DMG, p. 128), so the Intelligence of the thief doesn’t limit the level of spells .
The level that rangers and paladins cast a level 1 spell can be a bit confusing (“Does my Level 9 paladin cast Level 1 cleric spells as Level 1, or Level 9?”) but there’s no such question for scrolls. Unless specified otherwise, the scroll will function as a caster 1 level higher than required to cast the spell for the associated class, but at least 6th level (DMG, p. 128). A fireball (Level 3) can be cast as a level 5 magic-user, so will cast as if by a level 6 magic-user (which is also the minimum, i.e., Level 1 and Level 2 magic-user spells will also cast as if by a level 6 magic-user).
So the only thing left is “There is a 25% chance that writings will be misunderstood, and [an increasing chance of a reverse effect].” We have only one source for chances of magic spell failure:
DMG, pg. 128: Magic Spell Failure: If a spell-user acquires a scroll with a spell(s) of a level(s) not yet usable by the character, the spell-user may still attempt to use the spell; the chance of failure, or other bad effect, is 5% per level difference between the character’s present level and the level of magic use at which the spell could be used. [Any score that represents failure then uses the following table]
|Level Difference||Total Failure||Reverse or Harmful Effect|
|16 and up(a)||30%||70%|
(a) Aside: Why isn’t just 16-17? Even assuming 0-level casters, 16-18?
For other spell-users, the chance of magic spell failure is “5% per level difference between the character’s present level and the level of magic use at which the spell could be used”. For the thief, it’s a flat 25% (“there is a 25% chance that writings will be misunderstood “). That makes a strange sort of sense, since the thief doesn’t have a difference in “level of magic use at which the spell could be used” since it’s always Level 10. The implication of Magic Spell Failure is that failure is determined at the time of reading the scroll—the thief won’t know if they will be successful until they attempt to use the scroll. And once they read the scroll to release its magic (successful or not), the writing completely and permanently disappears from the scroll (DMG, p. 128).
But that still leaves the question of how to determine the “increasing chance per level of the spell that it will be the reverse of its intent.” The Magic Spell Failure Table uses the Level Difference to determine the chance of a reverse effect. However, all levels of spell are theoretically accessible by the thief; at 10th level they can utilize “scrolls of all sorts”. Are we back to the paladin/ranger conundrum? However, the thief’s chance of reversal is tied to spell level, not character level. We also don’t consider the thief level for chance of failure—why would we use it for chance of reversal?
Now with no By-the-Book reference, how then to determine the chance of reversal? The most extreme chance of reversal on the Magic Spell Failure Table is 70%. We can expect that even level 1 spells have a chance of reverse effect for the thief. Using the Magic Spell Failure Table for inspiration, we can come up with several methods to determine the chance of a reverse effect upon failure:
|Spell Level||Method: Level Difference||Actual Reverse Frequency||Method: By 5%||Actual Reverse Frequency||Method: By 10%||Actual Reverse Frequency|
Actual Reverse Frequency (above) is the calculated likelihood of a reverse effect occurring each time a thief uses a scroll (e.g., 25% chance of failure × 10% chance of reversal = 2.5% chance of the reverse effect occurring). From there we have to consider what was intended. Should a thief have, worst case, a 3.75% chance of a reverse effect when using a 9th level scroll? Probably not. 11% still seems too low. But a 23% total chance of a 9th level spell occurring in reverse seems reasonable, and comes close to maxing out the base chance of spell reversal at 90% (and appropriately worse than the 70% for reverse effect for the Magic Spell Failure Table). That leaves us with the following table:
Thief Spell Failure
Chance of Failure
Chance of Reversal
Now that that’s settled (inasmuch as it can be), should the thief be able to cast spells from any spell book? “In extremis, the DM may allow a magic-user to cast a spell directly from any sort of spell book just as if the book were a scroll” (UA, p. 80). The thief can only explicitly “utilize scrolls”, and the ability to cast a spell directly from a spell book is an ability granted to the magic-user “as if it were a scroll”—not that spell books are scrolls. So no.
For assassins, tertiary functions of assassins are the same as thieves, performing thieving at two levels below their assassin level (PHB., p. 29; UA, p. 13). So 12th Level assassin functions the same as a 10th level thief. And, like the thief, their ability to read scrolls never improves.