Rogue Archetype: The spellthief

[jiriku]

D&D 3.5 introduced a class called the spellthief in the Complete Adventurer's Handbook. I liked the idea behind the class, but it didn't compare in power level to most existing 3.5 classes so it wasn't very effective or popular. Here I'm trying to breathe new life into the spellthief by reimagining it as a 5e rogue archetype. Please offer your criticism and feedback!

Spellthief

Magical Training
When you reach 3rd level, you gain proficiency in the Arcana skill.

Spell Sense
Upon reaching 3rd level, you become sensitive to magical auras. As an action, you can open your awareness to detect nearby magical forces. Until the end of your next turn, you know the location of any magical aura (including magic items, active spell effects, and creatures able to cast spells) within 60 feet of you that is not behind total cover. You know the school of magic for each aura and the maximum spell slot level the item or creature can produce.

If you are 14th level or higher, you also learn the exact spells a magic item is able to produce or the spells a creature knows or has prepared.

You can use this feature a number of times equal to 1 + your Intelligence modifier. When you finish a long rest, you regain all expended uses.

Spellcasting
When you reach 3rd level, you gain the ability to cast spells.

Cantrips. You learn three cantrips of your choice from the wizard spell list. You learn another wizard cantrip of your choice at 10th level.

Spell Slots. The Arcane Trickster Spellcasting table in the Player’s Handbook shows how many spell slots you have to cast your spells of 1st level and higher. To cast one of these spells, you must expend a slot of the spell's level or higher. You regain all expended spell slots when you finish a long rest.

Spells Known of 1st-Level and Higher. You know three 1st-level wizard spells of your choice, two of which you must choose from the abjuration, divination, enchantment, illusion, and transmutation spells on the wizard spell list. The Spells Known column of the Arcane Trickster Spellcasting table shows when you learn more wizard spells of 1st level or higher. Each of these spells must be an abjuration, divination, enchantment, illusion, or transmutation spell of your choice and must be of a level for which you have spell slots.

The spells you learn at 8th, 14th, and 20th level can come from any school of magic.

Whenever you gain a level in this class, you can replace one of the wizard spells you know with another spell of your choice from the wizard spell list. The new spell must be of a level for which you have spell slots, and it must be an abjuration, divination, enchantment, illusion, or transmutation spell, unless you're replacing the spell you gained at 8th, 14th, or 20th level.

Spellcasting Ability. Intelligence is your spellcasting ability for your wizard spells, since you learn your spells through dedicated study and memorization. You use your Intelligence whenever a spell refers to your spellcasting ability. In addition, you use your Intelligence modifier when setting the saving throw DC for a wizard spell you cast and when making an attack roll with one.

Spell save DC = 8 + your proficiency bonus + your Intelligence modifier

Spell attack modifier = your proficiency bonus + your Intelligence modifier.

Steal Spell
At 3rd level, you learn to steal spells. When you hit a creature with a sneak attack, you can steal a bit of magical power from it. If the target is willing, you can use your action to steal with a touch instead. You cannot steal spells, spell slots, or spell effects whose level is higher than half your level in this class. Choose one of the following:

• You steal any spell the target knows or has prepared, or you can steal a random spell. You can’t steal a spell if the target is currently unable to cast it. The target cannot cast that spell for one minute and you gain the ability to cast the spell once without expending a spell slot within the next hour. The maximum number of stolen spells you can retain is equal to your Intelligence modifier (minimum one).
• You steal a spell slot. The target loses its highest level available spell slot (limited by the maximum level you are able to steal) and you recover one of your expended spell slots of that level or lower.
• You steal an active spell effect currently affecting the target. You can choose which spell to steal or steal a random spell. You gain the stolen effect for one minute (and the target loses it). If it required concentration, you must concentrate on it. After one minute, or when you stop concentrating, the spell it returns to its previous target. You cannot steal any spell effect whose duration is permanent.

Steal Resistance
At 9th level, you learn to steal resistance from a creature that has resistance or immunity. To steal resistance, you must hit a creature with a sneak attack. If the target is willing, you can use your action to steal with a touch instead. You can steal a resistance of your choice or a random resistance. You can steal resistance to one damage type (such as fire or nonmagical slashing attacks) or resistance to magic. You gain the resistance for 1 minute, and the target loses it (targets that have immunity remain immune).

Absorb Spells
Beginning at 13th level, when you successfully save against a spell that targets you, or when you counter or dispel a spell with counterspell or dispel magic, you can use your reaction to steal the spell (if you absorb a countered spell, this is done as part of the same reaction used to cast counterspell). You can only absorb spells whose level is no higher than half your level in this class.

Reactive Absorption
Beginning at 17th level, when you absorb a spell, you can immediately cast the stolen spell or a different spell that you know as part of the same reaction used to absorb the spell.

[theVoidWatches]

Seems very heavily loaded at 3rd level.

When you steal a spell with Steal Spell and gain the ability to cast it once within the next hour, do you need a spell slot to cast it?

What happens if you steal an effect that the target was concentrating on, then stop concentrating on it after they've cast a new concentration spell?

I suggest that if you steal an immunity, you gain resistance to that damage and the target's immunity is reduced to resistance.

Nonmagical attacks isn't one damage type - it's three (nonmagical bludgeoning, nonmagical slashing, nonmagical piercing).

Can you steal multiple resistances at once?

When you absorb a spell such as fireball, does it still effect other targets?

Absorb Spells can't work with Counterspell, because both require a reaction.

Does Reactive Absorbtion work only with Absorb Spell, or does it work any time you steal a spell?

[Nifft]

Since Arcane Trickster is already a thing, I'd suggest doing a Rogue-features archetype on top of a caster chassis -- either a Ranger, or a Sorcerer, or even a Monk.

Bard might be a good chassis if you're looking for a performer spellthief, but the sneaky kind seems more thematic to me.

[Phhase]

Whew. This looks really sweet. This kind insane flexibility/utility based on your environment, with a low base power level is EXACTLY the kind of thing I like.

[jiriku]

Seems very heavily loaded at 3rd level.

Yeah, it is kinda. I'm not sure how to correct that. Spell stealing really doesn't work well without sensitivity to magic, and sensitivity to magic plus the ability to cast spells doesn't make much sense without Arcana proficiency. I really have trouble telling the player to wait until 9th level for any of these.

When you steal a spell with Steal Spell and gain the ability to cast it once within the next hour, do you need a spell slot to cast it?

No. I will edit to clarify.

What happens if you steal an effect that the target was concentrating on, then stop concentrating on it after they've cast a new concentration spell?

I have no idea. I'd say that's a DM's call. At my table, I'd probably say the caster would have to stop concentrating on one of the two spells. And if I'm in the mood, I might force him to roll a concentration check and have Very Bad Things Happen if he failed.

Nonmagical attacks isn't one damage type - it's three (nonmagical bludgeoning, nonmagical slashing, nonmagical piercing).

Good point. I will clarify the example.

Can you steal multiple resistances at once?

Yes, without limit.

When you absorb a spell such as fireball, does it still effect other targets?

Fireball is not a targeted spell, so it could not be absorbed. But if you absorb a spell, the spell still has its normal effect -- note that if the spell deals a partial effect on a successful saving throw, you still suffer that effect. Should that change? For instance, I could make it more like the arcane trickster's spell thief capstone (which is an attempt to distill this entire class down to a single feature).

Absorb Spells can't work with Counterspell, because both require a reaction.

Good catch. I will write an exception to clarify that the countering and absorbing are done as part of the same reaction.

Does Reactive Absorbtion work only with Absorb Spell, or does it work any time you steal a spell?

Only with absorb spell.