Manna thief

You use the power of others against them. These rouges focus innate magical ability to manipulate any magic they cross paths with to harness its power for their own uses. Mage slayers, failed wizards, and adventures alike are some of the folks who tend follow this archetype.

Spellcasting

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 Transmutation, Enchantment, and Illusion spells on the wizard spell list.

The Spells Known column of the Spell Thief Spellcasting table shows when you learn more wizard spells of 1st level or higher. Each of these spells must be an Transmutation, Enchantment, or Illusion spell of your choice, and must be of a level for which you have spell slots. For instance, when you reach 5th level in this class, you can learn one new spell of 1st or 2nd level.

The spells you learn at 5th, 10th, 14th and 18th 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 Transmutation, Enchantment, or Illusion spell, unless you’re replacing the spell you gained at 3rd, 8th, 14th, or 20th level from any school of magic.

Spell steal

Starting at 3rd level, your innate magical abilities allows you to siphon off magical essence from your foes.

As a bonus action, when you have advantage on an attack against a creature you can steal the knowledge of a spell and how to cast it from that creature.  You can only steal a spell if the creature is able to cast spells and you have seen them cast a spell or you know prior that this creature can cast spells.  Stealing a spell does not remove that spell from the creature's spell list or a spell slot from the creature. A stolen spell must be of a level you are able to cast.  A manna thief can choose which spell they steal, one in which they have seen the caster use, or they can let the DM can determine the stolen spell randomly.  You can hold 1 spell using this ability and it is held for 10 minutes or until expelled by using one of the following options. 

The amount of spells you can hold goes up by 1 at levels 8, 13 sand 17.

Copy Spell
As an action, cast the spell as if you had it prepared using your spell save DC or spell attack bonus.  This ability consumes a spell slot appropriate to the level you cast it at.
Magic Armor
As a bonus action, expel one held spell to gain 1d6 temporary hit points per base level of spell that is expelled.
Channel Energy
As a bonus action, gain 1d6 elemental damage of the spell per base level of spell expelled on the next weapon attack. If the spell expelled does not normally deal damage the extra damage is counted as force damage.

Magic reflection

Magical Reflection

At 9th level, on a successful saving throw against a magical effect, you send some of its magic power towards its source. You can chose to deal damage to the originator the effect equal to your sneak attack damage whenever you succeed on saving throw against a spell or other magical effect. This ability only applies if the magic originates directly from a creature, (i.e. magical traps won't affect the caster if tripped).

Greater spell steal

Greater Spell Steal

At 13th level, whenever you use your spell steal ability the creature you steal a spell from loses a spell slot of that spells level or a use of the spell if applicable. Spells can be held for up to 1 hour or until expelled.

Siphon spell

Siphon Spell

At 17th level, you gain the ability to magically steal the knowledge of how to cast a spell from another spell caster.  When you use your Spell Steal ability against a spell caster, you may force them to make an intelligence save against your spell save DC.  On a failed save, they forget the spell you stole and it is removed from their spell list and you add it to your known spells.  You can have up to 4 spells known in this way. If you use this ability while you already know 4 spells in this way you must choose one of those spells and lose it replacing it with the new spell. The creature you stole the lost spell from then regains their knowledge of the spell if they are still alive.