My issue with shadowblade as it stands is the spell can be combined with another spell of equal or greater power that DOES require concentration that by itself is extremely powerful, coupled with the stone its game breaking.
No, it's not. Is using Ice Wall against wet enemies gamebreaking? How about Chain Lightning? Surely Bhaalist Armor must be with potential damage easily exceeding 200 for a single hit. No, I don't think any of those are gamebreaking. Nor do I think Resonance Stone and Shadow Blade are.
Again, I do think upcasted Shadow Blade is a bit much, and I could possibly be persuaded that 2d8 is a little strong, but the fact remains that no one uses Shadow Blade Ring because the concentration requirement is too big of an ask for a melee weapon. I wouldn't care if it was 100d100 base damage, concentration to keep a weapon is a terrible idea in the context of BG3. It would not be worth the frustration. Hence, Larian removing it from the new version.
Shadowblade with concentration would require risk vs. reward using the spell would still be powerful just not overwhelming so.
I wouldn't consider the privilege of having a melee weapon a worthy reward.
The duration would still last until LR just with the concentration component, any character using this spell should have a way of protecting their concentration, have a conventional weapon as backup if concentration drops, or just cast the spell again as a bonus action on the next turn.
You're either pregnant or not, there is no middle ground. Guard rails for concentration is just a lack of concentration requirements with extra steps.
My suggestion of necrotic was purely so it wouldn't get a benefit from the resonance stones psychic vulnerability, it could just as easily be any other damage type force damage could work just call the spell forceblade.
It's fine. I'd honestly like to have pure elemental weapons of all kinds in the game. I don't think it's(changing the element of Shadow Blade) necessary, either. Again, Steeped in Bliss has a significant penalty that cannot be decoupled from it's boon.
Again, I think the real issues with Shadow Blade are the power spikes at level 3 and the combination of upcasting with the Resonance Stone. If it requires a higher level and can't be upcast, those issues go away.
