.
Level
Base Attack
Fort Save
Ref Save
Will Save
Special
Spells Per Day
Xanathar Guild Spell Thief
1st
+0
+0
+2
+1
Sneaking Spell 1/Day, Trap Sense +1
-
2nd
+1
+0
+3
+1
Sneak Attack +1d6
+1 Level/Sorceror
3rd
+2
+1
+3
+2
Improved Uncanny Dodge
-
4th
+3
+1
+4
+2
-
+1 Level/Sorceror
5th
+3
+1
+4
+2
Slippery Mind
-
6th
+4
+2
+5
+3
Sneak Attack +2d6
+1 Level/Sorceror
7th
+5
+2
+5
+3
Improved Evasion, Sneaking Spell 2/Day, Trap Sense +2
-
8th
+6
+2
+6
+3
-
+1 Level/Sorceror
9th
+6
+3
+6
+4
Opportunist
-
10th
+7
+3
+7
+4
Sneak Attack +3d6
+1 Level/Sorceror
11th
+8
+3
+7
+4
Scribe Scroll
-
12th
+9
+4
+8
+5
-
+1 Level/Sorceror
13th
+9
+4
+8
+5
Defensive Roll
-
14th
+10
+4
+9
+5
Sneak Attack +4d6
+1 Level/Sorceror
15th
+11
+5
+9
+6
Sneaking Spell 3/Day, Trap Sense +3
-
The Spell Thief mixes the skills of a conventional rogue with the arcane power of a sorceror, to help himself on 'the job'. Most arcane casters consider what spell thieves do to be a perversion of the art and generally try to avoid them. Rogues on the other hand are intimidated by them and do their best to not get on the bad side of a spell thief.

Spell thieves use very little in the way of offensive magic, they prefer to use spells that can help them hide, speed, and get away from danger fast.
Detect Traps, Dimension Door, and Teleport are some of their favorites. Though they do usually learn the more common evocation spells. They will work with other to complete a goal, but prefer to work alone.
Hit Die: d6
Requirements
To qualify to become a spell thief, the character must fullfill following criteria.

Alignment: Any non-lawful
Base Attack Bonus: +3
Skills: Knowledge(Arcana) 8 Ranks, Spellcraft 8 Ranks, Use Magic Device 8 Ranks
Spells: Must be able to spontaniously cast 1st level arcane spells as a sorceror.
Special: Sneak attack +2d6, must be a member in good standing with the Xanathar Guild, must have used magic to burgle.
Class Skills: The Spell Thiefs Class Skills (and the key ability for each skill) are Appraise (Int), Balance (Dex), Bluff (Cha), Climb (Str), Concentration (Con), Craft (Int), Decipher Script (Int), Diplomacy (Cha), Disable Device (Int), Disguise (Cha), Escape Artist (Dex), Forgery (Int), Gather Information (Cha), Hide (Dex), Intimidate (Cha), Jump (Str), Knowledge (arcana) (Int), Knowledge (local) (Int), Listen (Wis), Move Silently (Dex), Open Lock (Dex), Perform (Cha), Profession (Wis), Search (Int), Sense Motive (Wis), Sleight of Hand (Dex), Spellcraft (Int), Spot (Wis), Swim (Str), Tumble (Dex), Use Magic Device (Cha), and Use Rope (Dex).
Class Features
All of the following are class features of the spell thief prestige class. 

Weapon and Armor Proficiency: Spell thieves do not gain any additional weapon or armor proficiencies.
Spells: A spell thief gains spells per day as a sorcerer, but he may only select spells from the following lists:   

Level 0: Acid splash, daze, detect magic, detect poison, flare, ghost sound, light, mage hand, open/close, read magic.   
Level 1: Alarm, animate rope, detect secret doors, disguise self, expeditious retreat, feather fall, identify, jump, mage armor, sleep.   
Level 2: Alter self, arcane lock, darkness, detect thoughts, invisibility, knock, locate object, see invisibility, spider climb, web.   
Level 3: Daylight, dispel magic, fireball, gaseous form, illusionary script, Leomund's tiny hut, lightning bolt, major image, shrink item, tongues.   
Level 4: Arcane eye, charm monster, confusion, dimension door, enervation, fear, greater invisibility, locate creature, polymorph, shout.   
Level 5: Bigby's interposing hand, fabricate, Leomund's secret chest, passwall, seeming, sending, telekinesis, teleport, transmute mud to rock, transmute rock to mud.  
Level 6: Antimagic field, chain lightning, contingency, globe of invulnerability, greater dispel magic, guards and wards, legend lore, mislead, shadow walk, true seeing.   
Level 7: Arcane sight, forcecage, greater teleport, phase door, power word blind, prismatic spray, reverse gravity, spell turning, statue, teleport object.   
Level 8: Bigby's clenched fist, demand, discern location, greater shout, mass charm monster, mind blank, moment of prescience, prismatic wall, protection from spells, screen.   
Level 9: etherealness, foresight, freedom, gate, imprisonment, prismatic sphere, refuge, shapechange, teleportation circle, time stop. 
Sneaking Spell: When the spell thief makes a successful sneak attack, he can choose to discharge one spell as a free action. This uses 1 of his spell slots for the day, but also increases the spell's save DC by 1. At 7th level this ability can be used twice per day and the DC bonus increases to +2. At 15th level this ability can be used three times per day and the DC bonus increases to +3. Creatures or persons immune to sneak attacks are also immune to this. 

Trap Sense (Ex): As the rogue ability. 

Sneak Attack: As the rogue ability. This stacks with any existing sneak attack ability. 

Improved Uncanny Dodge (Ex): As the rogue ability. 

Slippery Mind (Ex): As the rogue special ability. 

Improved Evasion (Ex): As the rogue special ability. 

Opportunist (Ex): As the rogue special ability. 

Scribe Scroll: At 11th level, the spell thief gains the Scribe Scroll feat.

Defensive Roll (Ex): As the rogue special ability.
Xanathar Guild Spell Thief (Prestigue Class)
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