Hello, I’ve been playing around with the idea of giving my sorcerer PC a damage threshold as a way to give the character some much need protection. The idea is to create the effects of Gojos infinity from jujitsu kaisen. Mechanically it would be a damage threshold of Charisma modifier plus proficiency bonus. Would love to know people’s thoughts on this and if it balances correctly.

Thank you 

You could get something similar with a home-brew magic item, like an amulet of false life that lets you cast "False Life" for free once per short/long rest.

The temporary hit points would work a lot like a damage threshold but there are already mechanics in place for handling them and its clear how they work, and as different sources of temp hit points don't stack there is already balance there.

you can of course re-skin/ re-flavor to give more of a jujitsu kaisen theme if you wish while keeping the existing mechanics.

I would make the amulet require attunement. And you could periodically (like say every time the char have a proficiency increase) give them a side quest / event that upgrades the amulet to cast false life at a higher level to keep it useful as they level. Or you could not bother at the higher levels char usually aren't running out of spell slots and cast their own buffs.

I’d say no. Lower hit points and ac is a game balance strategy for casters. Sorcerers can cast fireball and lots of other very powerful spells, but they have lower hit points. That’s the trade off.
If it’s really a problem, that’s why there’s spells like mage armor and shield. 

I really like this idea, i should have put this in the original post but the current plan is to tie the ability to the character innate sorcerer (One dnd) so when they use the ability they receive this buff. I will definitely look into the false life route. 

Agree with the others, it's very much not balanced. Sorcerers are supposed to be vulnerable so that they have to choose between using their precious spellslots for defense or offense. It the sorcerer wants more protection they can use Mage Armour, Shield, Protection from Good/Evil, Absorb Elements, Blur, Mirror Image, Haste, Blink, or other spells to improve their defenses. Whereas if they want to use those spellslots & actions to deal damage or control enemies they must sacrifice their defenses to do so. That's the balance of casters.

I would give the the following magic item, which is a modified Elven Chain with the feature from the Heavy Armour Master:

  Name it whatever you like

For reference, that offers 14 +Dex modifier (max 2)

Get a feat so you can wear armor.

Multiclass so you can wear armor.

Use spells that increase your AC.

Realize that you're probably not going to solo everything and depend on the rest of the party to keep you out of trouble.

Chasing higher ACs is a viscious cycle that characters almost always lose. A better tactic would be to find a better tactic to avoid getting close enough to be hit. If the average AC in the group goes up, the DM will create encounters using monsters with a higher to hit bonus to keep things challenging. But those higher to hit bonuses come with much higher damage dice. At certain levels/hd a +1 increase in to hit bonus equates to a 20pt increase in damage per hit. Look up monsters with a +6 to hit, and compare their damages to those with a +7 to hit, or even +8 and you'll begin to see what I'm talking about.

Whereas, if you keep the average AC of the group low, the DM will add larger numbers of monsters with lower to hit bonuses to the encounters to keep them challenging as you level up. You'll still get hit at the same rate, but you'll feel those hits a lot less.

It's funny to watch PCs that go insane for the absolutely highest ACs possible. Yes, there's quite a few fights where they don't get hit at all... until they face a creature with a to hit bonus high enough to reliably hit them, then they suddenly realize the damage per hit they now have to take is more than their hp total can withstand.

Between Blur and Shield, a sorcerer's AC should be just fine without any additional help.

Don't give them this magic item. Here's how it'll go:

DM: "You find a magic item which gives you a damage threshold equal to your charisma bonus plus proficiency!"

Party: "Oh, you should give that to the barbarian! Then he can take double that while raging and take no damage!"

Sorc: "Oh, that would be awesome!"

DM: "Oh bugger."

Sorcerers are squishy. If they are getting hit, they're in the wrong place. If they keep getitng hit and refuse to take defensive spells, they are in the wrong trade. 

If you did damage reduction like i suggested, that is calculated before resistance so wouldn’t really be that helpful for a Barbarian

If you want to have that kind of ability you should play a different game that has such a thing built into the rules - GURPS or Fantasy HERO.

If you insist on this thing in D&D, one you are going to make a lot of work for yourself as the GM. You could also consider using Temporary HP for such a mechanic.

I feel like there is a lot of naysaying for something that is OP’s judgmental call as DM. Just make a magic item that can only be attuned by sorcerers if you are concerned about munchkinery.