Couple of thoughts. First I echo other's concerns about this on multi-target spells.
The OP specified that it only works on spells that target one creature.
Couple of thoughts. First I echo other's concerns about this on multi-target spells.
A 6th-level wizard casts a fire bolt at a fleeing enemy 100 feet away, swaps out both damage dice to restrain the target. The flames deal no damage but...uh... somehow wrap around the target and bind it in place. Huh, that seems a little weird narratively – fire ensnaring a creature? and dealing no damage while wrapped around them?
But wait! How long does the restrained condition last on the target? Is it concentration, up to 1 minute (which is typical)? Then we've just allowed out wizard to replicate entangle (a 1st level spell) at a greater range than entangle itself can accomplish using only a cantrip!
Upon attaining 7th-level, the wizard learns and casts wall of fire in a battle against a solo villain, hoping to trade out 4 of the damage dice when a creature passes through to blind it. However, because the wall of fire doesn't have a target, this feat cannot be used with that spell. Even though the narrative makes perfect sense.
I feel like this falls into the trap may homebrew feats do: the story behind it doesn't serve to further clarify the character's theme/personality/style.
I mean vaguely it's a "control" mage, but "control" really doesn't correspond to anything in the fiction.
An opposing example would be back in the playtest that had "implement feats" (IIRC one was called Golden Wyvern Adept or something) which were to spellcasters kind of what like Crossbow Expert, et cetera, are to warriors.
There's also a red flag right there in your title: Enhanced Spellcasting. Who wouldn't want Enhanced Spellcasting? So many characters in this game have spellcasting ability. This would be so hugely attractive that it would start to edge into that territory of "must take." And that's something to be avoided in feat design.
EDIT: Not to leave with just a critique, but to contribute something positive, I'd suggest starting with the story first...
For example, let's say True Names play a significant role in your game – not just of fiends but of other fey and other spirits. You might devise a True Namer feat that grants the PC benefits to spellcasting when it knows a creature's True Name, and conversely can be used to help a friendly creature whose True Name is known to the caster shake off charm/possession/shapeshifting effects.
Another example, let's say you have a player who loves to blow stuff up with magic – I've heard those types of players are out there.You might create a Concussive Magic spell that allows them to upcast evocation spells in a way unique to the feat, forcing Strength/Constitution saves to remain standing and not be pushed backward and/or knocked prone. You could even include bits about dealing extra damage to untended objects to simulate blowing stuff up. Possibly give them a reaction feature to absorb kinetic energy that would knock them prone/push them, requiring the expenditure of spell slots to use.
Being able to paralyze anytime you cast a reasonable damage spell in exchange for 21 (or even just 15) damage is not really balanced. I would *always* do that when casting any single target damage spell of 6 dice or more.
You'd also need to specify how the blind/paralyze/etc... ends.
I think this would be better handled by not introducing a feat, but instead introducing spells they can prepare that do these things. For example, a 3rd level spell that deals 2d6 damage, and continues to do a d6/ round and paralyzes certain types of creatures on a failed save (save ends both) would be reasonable.
Also, don't forget as a balancing mechanic that you don't have to give away the farm in a feat; you can always say, "+1 to {STAT} and plus the following effects:" That way, you can limit the scope of the extra powers a bit more and not feel like you have to give tons of abilities to make a feat worthwhile.
Thanks. That may be the answer I am looking for. Do you think going +1 int and one of the theee abilities makes it interesting, balanced and attractive?
Something like this:
Binding Spellcasting
Skilled in manipulating eldritch forces, you can replace some of the damage dice from a spell and use the mystical power to restrain the target.
Benefits:
- +1 to WIS, INT or CHA
- When you cast a spell with an instantaneous effect that causes damage and targets one creature you can swap out 2 dice of damage for the ability to restrain the target of the spell (spell DC to negate). The target can make a svaing throw at the end of it's turn to end the effect.
The target of the spell must take damage for the secondary effect to occur and maintaing the restraint requires concentration.
Personally I think that the feat put forward by the OP offers way too much flexibility to an already very flexible group of classes. 5e's prepared/known spell mechanic really offers caster the opportunity to be prepared for any situation.
If I were to offer feats that were specific to casters I would try to emulate the feats offered to martial characters, along the lines suggested by 5ekyu in post 5.
Things I would also think are more interesting.
Extra Spell slots this closely emulates Magic Initiate, which offers cantrips and one spell slot. So I would offer 2 level 1 spell slots. I think this would balance against MI by not having the cantrips and being from your own spell casting list. You could possibly allow this feat to be taken a second time to get one level 2 spell slot. Overall I think this is a REALLY powerful feat though, possibly unbalanced (looking at you variant human), so should probably be gated to level 5.
Extra Spells Prepared/known Much like Krachek's Expanded Mind idea. I think this could be a really good feat for classes with limited spells known but is reigned in by limited spell slots - you still can't cast any more spells.
Boost Spell Save DC Much like Krachek's Magic Sharpness idea. This could be a little OP but is kind of in line with the archery fighting style (+2 to hit) but more powerful than an ASI (+1 to save DC) since it almost gives you two ASI at once less the other benefits of your casting stat (like better saves for you).
Trade probable success for stronger effect something along the lines of -5/+10 offered by GWM and SS. A little harder to do for spells because of the varied effects, but you could do something along the lines of -2 save DC to upcast the spell with a lower spell slot. eg cast fireball with 9d6 damage from a 3rd level slot, but the targets all get a lower save DC. -2 seems a little low but -5 seems a little high and I'm always reluctant to have different modifiers to a d20 roll in 5e
For extra spell slots, you could have a prerequisite of being able to cast a 2nd level spell. That would keep it out of the variant human's hands until level 4 like all other races. That would end up delaying it for rangers and paladins. Also, I am not sure how it fits with warlocks--it seems like those 1st level spell slots would eventually become 5th level spell slots. That would be a pretty substantial benefit compared to other classes.