Spellthief

Spellthieves are specially trained to combat against or side by side with spellcasters, and using them to their advantage. Even though they can't cast spells on their own, they have ability to manipulate with magic cast by even the strongest of spellcasters. Be it copying, amplifying or even just releasing raw magic, Spellthieves always come at top when fighting near some magic source

Our Spell

As your action you can try to copy any of your target's spells on their spell list. Target must succeed 16 constitution saving throw, and if they fail, you can choose one spell from their spell list and use it immediately (for no action). You can't copy the same target two consecutive rounds. Spells are always used at their lowest level (cantrips are used with their level 1 power)

Notes: Your to hit with copied spells is your Dex score + Proficiency score

             You can't use this feat if you haven't used the copied spell yet

             You can use this feat to copy your allies' spells too

Leveling: Starting with level 5 you can store copied spell for one round. Upgrades to 2 rounds at level 10

Bonus Attack Action

When using your action for something different than attacking, you can use this feature to attack for no action

These attacks are rolled as if you used them as action (no nerfs)

You can use this feature times equal to your proficiency bonus

Mana Empowering

When there is any spellcaster within 30 feet of you, your weapon attack can deal additional 2d4 radiant damage, and heal you for half of damage dealt with this feat. Is stackable with sneak attack, and also applies on reactions like opportunity attack, sentinel strike, etc...

You can use it only once per turn +reaction (so two times per round at most)

Magic Amplification

When somebody within 30 feet of you uses spell, you can use your reaction to empower it by 1 additional dice it uses (so if spell would originally deal 2d6 damage, it will deal 3d6, or if it heals 1d8, it will heal 2d8, etc...)

Upgrades to 2 dice at level 17

Works also on spells cast from magic and cursed items

Mana Blast

As action you can suck out magic energy of the world itself and release massive blast dealing 5d10+[dex modifier] damage to everything within 20 feet of you (including allies). After you use it you must wait 2 rounds to use it again (meaning that when you use it, you can't use it for next 2 turns), but struck creatures have disadvantage on their attack rolls for next round

Note: Damage type differences depending of surroundings. Read this feat's action info to find out what damage will it deal