The Arcane Thief, had adapted themselves to have actions, bonus actions and reactions that mystify the very workings of magic.  When asked, the few who do exist can't really explain it, but do say it seems to be an application of will or need in which they steal the actual magical manifestation of real casters. That said, the few who have been found seem a bit tight lipped.  Granted, they are rogues who are not quick to spill their secretes anyway, but when asked, the look in their eye changes, maybe it is fear or frustration, maybe it is a promise kept, maybe it will never be known.

Rogues as a secretive sort in general, have left people wondering about the Arcane Thief. it may be a select variation of the Arcane Trickster, but they seem much less common. There may be 10 or 100 or 1000 in the world. The tales told tend to lean toward one common thread.  Very few who come against the Arcane Thief seldome live to tell the tale.  There is even more secretive rumor that the magically inclined, be it arcane or divine, love them less and fear them more than nearly everyone else.

They have picked up and adapted the casting of magic and integrated it with their roguish abilities.

Spellcasting

When you reach 3rd level, you gain the ability to cast spells.  However, your spell casting is unlike any other class or subclass.  Upon reaching 3rd level and choosing this subclass, you come up on a magical spell book which becomes the source of your magic power.

Cantrips

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

Spell Slots

The Arcane Trickster does not follow a spell casting table because all of their spells are stolen and the only slots used are to cast spells they have made permanent in their book. Arcane thieves can not up cast spells. You regain all expended spell slots when you finish a long rest.

Spells Known of 1st-Level and Higher

You may have three spells of your choice written as permanent in your spell book when you choose this subclass at 3rd level but you will need to have spell slots available to cast permanent spells.  Remember the highest level spellslot an Arcane Thief can obtain is a 4th level slot. All spells stolen of a higher level can only be recast before written, and if written will cast as a scroll.

Whenever you gain a level in this class, you can replace one of the permanent spells you know with another spell of your choice from your current spell book. Additionally, you can choose a spell written in the book to become permanent as long as you have the level spell slot to cast it. Alternatively, you can save the permanent spell ability until you have stolen a spell you want to keep and recast using your spell slots.

Spellcasting Ability

Charisma is your spellcasting ability for your spells. You use your charisma whenever a spell refers to your spellcasting ability. In addition, you use your charisma modifier when setting the saving throw DC for a spell you cast and when making an attack roll with one.

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

Spell attack modifier = your proficiency bonus + your charisma modifier

 

The Book

The Magic spell book you come on, and are somehow bound to when you choose this sublass at 3rd level will be your partner the rest of the game.  How your come up on it is up to you (That means write it into your backstory...and make it good...no one likes a boring backstory)

The book has the following features.

1. Your book is self animated and self willed.  It will take all actions to remain within your 3' personal space aura at all costs.  Should it be stolen or destroyed (AC of 30 and 5 hitpoints per spell level it currently contains) it will reappear in the space adjacent to you in 1d4 hours.

2. Your book contains room for 30 spells and 4 cantrips...the book itself adds and deletes pages as needed to maintain this ability. As you grow in wisdom and ability you will be able to make perferred spells permanent in your book...meaning they will not disappear when you cast them.  Your book will allow you 1 permanent spell per skill level.

3. Your permeant spells will use a spell slot as provided to your by your association with the Arcane Trickster subclass. Spells which are not permanent but written into your spell book will disappear when cast as does the script on a spell scroll.

4. You can maintain 3 spells in memory at any given time...however if you steal a 4th spell, then the oldest spell in your memory will go off next turn, whether you are smart enough to cast it or not.  If you did not, or were unable to cast it for any reason, it will cast itself on you.  This information is given to let you know that scribing memorized spells into your book is very important.

5. Spells can be scribed into your spell book with normal Quill and ink. It takes 10 minutes to scribe a single spell level into your book...therefore it would take you an hour and a half to scribe the wish spell.

Steal Spell

At 3rd level the Arcane Thief has the ability steal the arcane or divine magic others are using to cast spells. The arcane thief can use their action, bonus action or reaction to steal leveled spells in 3 ways.

1. They can use their action or bonus action touch attack on a willing or unwilling target. If it is a willing target they can steal a single spell from that target of a level up or their charisma modifier + proficiency bonus. If the target is unwilling the touch attack must exceed their armor class and the target can make a charisma save vs caster spell save DC.

2. During a sneak attack the Arcane Thief can choose to forgo their sneak attack die damage to instead steal a spell up to their charisma modifier + proficiency bonus.

3. As a reaction, the arcane thief can attempt to steal the energy of a spell cast at them as a target or an area effect spell in which they are in the area of effect. When the spell is cast, the arcane thief must use their reaction to roll a performance check to duplicate the spellcasters actions in reverse. If the total rolled exceeds the casters level + the spell level then the spell is absorbed and stolen. The effect of the spell does not happen, and the Arcane Thief retains the ability to recast the spell.

The arcane thief can only retain three leveled spells at a given time. Cantrips are permanent and are not effected by this rule. If a fourth leveled spell is absorbed, the arcane thief must immediately cast one of the retained spells in their next turn or the oldest retained spell will cast itself with the arcane thief as the target.

Casting the spell requires none of the material components of the spell because they were already invested in the energy. The Arcane Thief must perform the verbal and somatic components of the spell backwards again with a performance check based on the original casters level + spell level. Success allows them to cast the spell at the original casters level. Failure means the spell fizzles and is lost. In the event an enemy wants to counterspell the Arcane Thief's spell, they must first roll an Arcan Check at -5 against the Aarcane Thieve's spell save DC because the components of the spell cast by the Arcane Thief are all reversed.

The Arcane Thief can copy retained spells in their spell book at the rate of 10 minutes per level of the spell being copied. This is done with any regular Quill and  ink.

This book becomes entwined with the Arcane Thieves energy. If it is stolen, or destroyed, it will magically reappear in a spot adjacent to the arcane thief in 1d4 hours.

Sneaky Spell Attack

When you have advantage on an attack or the enemy you are attacking has another party member within 5' of them, you can choose to trade your sneak attack to cast a cantrips or spell which requires an attack roll.  You roll the attack roll with advantage as with a sneak attack and add your sneak attack dice if you hit.  The damage type of the sneak attack dice becomes that of the spell.

When you use the spell sneak attack you may choose to use your charisma modifier rather than your strength or dexterity modifier, and your charisma modifier is added to the damage.

Embraced by the Arcane

Starting at 9th level, your extensive interaction with magic has become a part of your very being. Your Charisma bonus are added to your initiative and AC

Absorbed of the Weave

At 13th level, you gain the ability to allow the Weave of magic to absorb you.

As a bonus actions you can offer both your actions and reactions to another creature. You give up your actions and reactions and disappear from existence until the beginning of your next turn, designating one creature of your choice to be given your action and reaction for the turn.  They can take this turn immediately or wait until their initiative to do so.  If they have not used the action and reaction before the beginning of your next turn, you are returned to the same spot or closest unoccupied position and the unused action and/or reactiont are lost.

Steal Resistance & Immunity

Beginnig at 17th level, you can siphon off a target's resistance to any damage type. When you hit an opponent with a successful sneak attack, you can choose to forgo dealing 3d6 points of sneak attack damage and instead temporarily gain resistance or immunity to an energy type to which your target is resistant or immune. If the target is willing, you can steal energy resistance with a touch as an action.

Simultaneously, the target creature's resistance to that energy type is removed.

If your target has more than one type of resistance to damage, you can choose which kind to steal; otherwise, the DM determines the stolen resistance randomly from among those possessed by the target. If you choose to steal a type of resistance that the target doesn't possess, the stolen type of resistance is determined randomly from those possessed by the target.

The resistance you gain from using this ability lasts for 1 minute. If the resistance is derived from a temporary effect (such as a spell), the stolen resistance disappears when the effect expires.

You can use this ability multiple times, but its effects do not stack unless they apply to different energy types.