Would this feat be overpowered?

[golentan]

I've always liked the idea of wizarding duels, squaring off and flinging spells at each other until one is unable to continue. Part of the fun is in the give and take, of preventing the enemy from landing their spells. As such, I homebrewed this feat for 5th and higher level casters to represent a focus in fighting other mages.

Magical Duelist
Prerequisites: Ability to cast Counterspell

Many mages compete to demonstrate their superiority in the magical arts, but you have raised it to an art form. As an action, you can enter or leave a Duelist's Stance, picking a single target as your opponent that you can see. For the duration of the stance, you gain the following effects.

1) Spells you cast can only target your opponent. Any spell you cast on a being other than yourself must have the opponent as the single target or position the opponent in the area of effect. Casting a spell or making a mundane attack against any foe other than your opponent immediately ends your Duelist's Stance.
2) You have disadvantage on saving throws against enemies other than your opponent, other foes have advantage when attacking you, and if you take no actions the turn before you gain advantage on saving throws against spells cast by your opponent.
3) Casting counterspell is less taxing of your magical resources. When using counterspell against your opponent, you regain an expended spell slot of a level lower than the one you expended on counterspell.
4). When casting counterspell, you can immediately cast a single cantrip you know as a bonus action.

[TheOOB]

Well first, the feat doesn't work. Counterspell is a reaction, and you can only use bonus actions on your turn. Ability #2 also is questionable how it would work with that rating. In particular, you don't get saving throws from people but effects. For ability 3, I'd say flat no, no feat should be able to restore an 8th level spell slot.

How about this feat instead

Magical Dualist
Prerequisites: Ability to cast 3rd level spells
* You know the Counterspell spell and can cast it once per day as a 3rd level spell without using any spell slots. Your casting ability for this spell is the same used by the class you used to qualify for this feat. You may cast counterspell additional times per day using your spell slots as if it was one of your spells known/one of your prepared spells(Counterspell does not count against your daily prepared spells/spells known).
* When you successfully use Counterspell, you gain advantage on your first attack roll against the caster of the spell you countered until the end of your next turn.
* You gain advantage on all ability score tests to counter a spell with Counterspell

[golentan]

Well first, the feat doesn't work. Counterspell is a reaction, and you can only use bonus actions on your turn. Ability #2 also is questionable how it would work with that rating. In particular, you don't get saving throws from people but effects. For ability 3, I'd say flat no, no feat should be able to restore an 8th level spell slot.

How about this feat instead

Magical Dualist
Prerequisites: Ability to cast 3rd level spells
* You know the Counterspell spell and can cast it once per day as a 3rd level spell without using any spell slots. Your casting ability for this spell is the same used by the class you used to qualify for this feat. You may cast counterspell additional times per day using your spell slots as if it was one of your spells known/one of your prepared spells(Counterspell does not count against your daily prepared spells/spells known).
* When you successfully use Counterspell, you gain advantage on your first attack roll against the caster of the spell you countered until the end of your next turn.
* You gain advantage on all ability score tests to counter a spell with Counterspell

I based the spell regain on a watered down version of the Divination class power to regain spell slots when casting Divination school spells. Also, to regain an 8th level slot you would have to have already spent a spell of 8th level and give up a 9th level spell on counterspell, which seems frankly wasteful. I figured the disadvantage against other foes is pretty clear: if you need to make a saving throw, and your opponent's action was not responsible for triggering it, you're disadvantaged.

[Orvir]

Why not play an abjurer? The temp hp, improved counterspell/dispel magic and spell resistance could make them pretty tough vs other casters.

[akaddk]

I like OOB's version better. Yours is far too complex and convoluted for 5e. OOB's is pretty straightforward and still quite useful and powerful. Given it's situational usage, I'd say it's in the ballpark of being the right level of power for a feat as well.

[TheOOB]

I based the spell regain on a watered down version of the Divination class power to regain spell slots when casting Divination school spells. Also, to regain an 8th level slot you would have to have already spent a spell of 8th level and give up a 9th level spell on counterspell, which seems frankly wasteful. I figured the disadvantage against other foes is pretty clear: if you need to make a saving throw, and your opponent's action was not responsible for triggering it, you're disadvantaged.

The diviner ability is a major class ability that requires to cast spells that generally have limited direct in combat benefit. Your ability is a feat which requires to cast a spell with huge direct combat benefit. A feat shouldn't be an (arguably) better version of a major decently high level class feature, especially as only part of it's effect.

The one is not worded in a way that works. If an opponent triggers a trap you're standing on, does it work? What if it's a summoned creature by that foe. What it it's a persistent effect they cast before you entered the stance? You didn't specify an effect.

In my version I tried to take the basic itea(being a spell caster who's really good at using fighting other casters via counter spell) and make it a littler simpler and more consistent with other feats. With Polearm expert you don't enter a polearm stance, you just get some cool abilities with Polearms, so I used that feat, and several similar feats as reference.