Arcane Thieves have an understanding of the energies of magic and have the unique ability to steal these energies from others. This means they have been given an unfortunate moniker “killers of magic.” However most Arcane Thieves simply study the arcane to enhance their own abilities or because they are interested in magic. That being said if your a spell caster and an Arcane Thief is on your tail - run! 

The tradition of Arcane Thievery supposedly started when a young initiate in wizardry was attacked by a lich who had foreseen that the young individual would kill them. The young wizard’s master died defending them and the youngster was found and raised by a master assassin. The child learnt to combine his magic abilities with those of a rogue and swore to one day have vengeance for their former teacher. And vengeance they got or so the story goes. Since that day the lich fell Arcane Thieves have been devoted to fighting the forces of dark magic but no one is ever truly immune to the darkness and many Arcane Tricksters have fallen to dark paths.

Arcane Knowledge

At 3rd level when you pick this archetype you gain proficiency in arcana and history checks made about magic the history of spells.

Spell Steal

Beginning at third level, you can use your knowledge of thievery and the arcane to steal arcane energy from other spellcasters. Immediately after you deal Sneak Attack damage to a creature you can make an intelligence check against the targets spell save dc. If you succeed you can steal the magic of one of their spells. The target is unable to use that spell for a number of hours equal to your intelligence modifier and for the same amount of hours you can attempt to cast the spell. To cast the spell you must make an ability check with your intelligence (dc = 10 + twice the spells level). You can cast each spell a number of times equal to your proficiency bonus per long rest.

Your spell save dc is equal to 8 + your proficiency bonus + your intelligence modifier 

You can use this ability to steal spells a number of times equal to your proficiency bonus per long rest.

 

Scrappy Spellbook

At 9th level you have emulated the spellbooks of wizards compiling scrappy notes of spells you have stolen in the past. The spellbook can have any appearance you desire or can appear to be a mixture of any spellbook of wizards you have stolen from. The spell book can contain a number of spells up to your proficiency bonus. These spells can be any you have stolen in the past.After a long rest you can pick new spells that you have previously stolen to replace the ones in your spellbook. You can cast each spell in your spell book a number of times equal to your proficiency bonus. Each time you attempt to cast a spell you must make an arcana check (dc = 10 + twice the spells level) if you fail the spells magic falters and has no effect.

Eldritch Thief

Across your many adventures and arcane studies you have come across many different kinds of magic and have unlocked some of their secrets. At 13th level you can pick one warlock pact boon and gain it. If you have already got a pact boon or are gaining a pact boon you cannot pick a pact boon you have already taken.

Block The Arcane

You have learnt how to block the flow of magical energy through creatures. Starting at 17th level when you deal damage with Sneak Attack you can force the target to make a constitution saving throw against your spell save dc. If they fail they cannot cast spells for a minute, If they succeed the next time you use Spell Steal you can steal an extra spell from them. Once you have used this ability you cannot use it again till you have finished a long rest.