When you select this path, you gain the following abilities:

Spellcasting

When you first choose this path, you gain access to the Spellthief Spell List and Mana casting mechanic as detailed below:

Starting at 6th level, if you successfully steal a given spell from unwilling targets three times, and cast it each time, you have a chance to master it by passing an Intelligence (Arcana) check, DC 10 + 2 per level. If you fail, you can try again the next time you steal and cast the same spell. If you succeed, and the spell is of a level you are capable of casting, you permanently add the spell to your list of spells known.

Starting at 9th level, you may now use Spell Steal to steal the supernatural abilities of creatures. This follows all the normal rules of Spell Steal, except that you steal a random supernatural ability, not a spell, and the DC is 10 + the challenge rating of the creature (+2 if you're "calling" a specific ability you know about).

You can steal passive abilities, such as regeneration or darkvision, or active abilities, such as a breath weapon. You cannot steal legendary abilities. If the creature has a given number of uses per day, and has used some of them, you only get the remainder; if they've all been used, that ability is not subject to stealing.

Abilities that you steal remain available to you for 1 minute per Spellthief level. The creature loses the ability for the same amount of time.

You cannot use Spell Mastery on monster abilities.

It is up to the DM's discretion what constitutes a "supernatural" ability, as opposed to a natural one, because WotC removed the distinction between extraordinary and supernatural abilities in 5e for some stupid reason. Generally, an ability is supernatural if it violates the laws of physics (discounting anything that is clearly a bodily function, such as claw or tail attacks, despite physical unlikeliness of certain monsters).

• Arcane Strike: Standard action, melee attack; you empower your attack with arcane power, causing it to inflict 1d8 bonus force damage.
  • Option: by spending 1 Mana, you can add 1d8 bonus force damage. No limit to how many points you can spend.
  • Alternate use: you can project your melee attack up to 30 ft, inflicting only the bonus force damage (no damage from the weapon itself). This still counts as a melee attack, and can be used to deliver a sneak attack.
• Bampf: Move action; you teleport up to 15-ft to a location you can see.
• Scan: Bonus action, 60-ft range; you scan one target, learning if it possesses magical abilities. Roll a Investigation check; on a result of 15 or higher, you learn the list of spells or magical abilities it has available (of up to the level you yourself can cast), and any resistance to magic it possesses. On a 20 or higher, you learn of any vulnerabilities to magic it might possess, and spells of any level it has available.
• Spell Lock: Reaction to an enemy casting within your reach; make a melee attack. If it hits, target must pass a Con save or lose the spell they are casting, as well as being unable to cast any spells of that school until they pass another Con save (minimum 1 round).
• Vanish: Reaction; you become invisible until your next turn. Attacking also ends the invisibility.

Assault

• Fan of Knives: Single action, close burst 10-ft; you project the force of your melee weapon many times over, causing dozens of force blades to radiate from you, inflicting 2d8 force damage to targets within 10-ft. (For each extra Mana spent, add 1d8)

Debilitation

• Sandman's Touch: Single action, melee range; a single creature, who is not alert to danger, falls asleep on a failed Con save (on success: no effect, and doesn't notice unless he passes a Perception check). Once asleep, he can only be woken by vigorous action, very loud noise, or physical damage, or until completing 8 hours of sleep.

Defense

• Spell Capture: Reaction to being hit by a targeted spell, Mana cost equals the level of the spell; you attempt to steal a spell just as it strikes you. Roll an Int save against the spell's DC; on failure, the spell affects you as normal, and on success, you steal the spell, and it does not affect you (alternatively, you nullify the spell, and are refunded the Mana cost of this spell).

Shaping

• Low Fog: Bonus action, 30-ft range; you create a low-hanging fog, spreading throughout a 30-ft by 30-ft area, but only from ground level to a height of 3-ft. Taller creatures can see each other, but everything beneath 3-ft is concealed. A medium-sized creature can stoop over to fit within the fog, but their movement speed is halved. Small creatures can move at full speed through the fog while remaining concealed.

Utility

• Legerdemain: Bonus action; you gain the ability to open locks, disarm traps, pick pockets, and otherwise manipulate with your hands within a range of 30 ft. Lasts as long as you concentrate.
• Steal Appearance: Bonus action, melee range; you steal subject's appearance. You are perfectly disguised as the subject as long as you concentrate.

Assault

• Void Strike: Single action, melee attack; as a normal melee attack, but also attempts to dispel up to 3 defensive spells before applying its damage. For each, roll opposed Arcana; if you succeed on a check, you dispel one random defensive spell and may continue trying (max 3 attempts). If you fail, you do not dispel, and are allowed no further attempts before applying damage as normal.
  • A "defensive spell" is any targeted spell currently affecting your target which could in some way reduce the likelihood of your attack causing damage, or mitigate said damage.
  • You need only score a hit against the subject's unarmored AC to roll dispel attempts. After all attempts, the hit must exceed actual AC to inflict damage.

Debilitation

• Spell Scrambler: Bonus action, touch range; using a modification of your Spellsteal technique, you scramble the spells available to the target. Each time they wish to cast a spell for the next 3 rounds, they must pass an Int save; on success, they cast the spell normally, but on failure, they instead cast a random spell from their repertoire.

Defense

• Cloak of Shadows: Reaction to a harmful magical effect; you gain 90% resistance to magical damage, as well as advantage to saves against magic, until your next turn.

Utility

• Antimagic Zone: Bonus action; the square you're standing in becomes an antimagic field (all magic is completely nullified while it remains in the area, no one can use magic while inside). Lasts 2 rounds.
• Spellstash: 1 minute ritual; you imbue some or all of your currently-stolen spells into an object. While inside the object, they are not lost with a long rest, nor are they accessible to you. You can retrieve a spell from a stash as a bonus action, but it cannot be put back in the same stash (although a newly-stolen copy of it can).
  • Maximum capacity of the stash (in terms of the sum of all spell slots' level) is based on the item's value: 10 gp, max is 5. 100 gp, max 10. 500 gp, max 20. 2000 gp, no limit.

Mobility

• Subjective Gravity: Bonus action; for you, the force of gravity applies in relation to the nearest stable surface. Thus, you can walk on walls and ceilings as easily as walking on the floor. Concentration.

Utility

• Hijack Spell: Bonus action, melee range, Mana cost equals the level of the spell; you target a spell currently in effect, and attempt to turn it over to your control. Roll an Int save against the spell's DC; on failure, nothing changes, but on success, you command the spell, and henceforth it behaves as if you had cast it. Thus, an ongoing spell that harmed your allies now harms your enemies instead; a beneficial spell for an enemy now becomes yours; an attack against you is turned on its attacker. (Probably needs some clarification)

More spells are planned...

Mobility

• Ethereal Jaunt: Move action; you step into the ethereal plane, becoming invisible and intangible to all within the material plane. You may then move up to a normal movement action, after which you reappear in the material plane. No material obstacle can block this movement, except for magical force effects.

More spells are planned...

• Spell that ties up an enemy using your own skill at Sleight of Hand
• Phantasmal Flanker
• Mass sleep