This mod adds the Vital Sacrifice feat from Tal'Dorei Campaign Setting Reborn to BG3, represented as faithfully as I can make it given game adaptation, limitations, and ambiguous wording.

Feat: Vital Sacrifice

You've learned secrets of hemocraft that grant you esoteric power at the price of your own life force. As a bonus action, you can choose to take 1d6 necrotic damage to gain a blood boon. Your blood boon lasts for 1 hour* or until expended.

You can expend this* blood boon to gain one of the following benefits:

• When you make an attack roll*, you roll 1d6 and add it to the total.
• When you hit with an attack or spell*, you deal an additional 2d6 necrotic damage.
• When you cause a creature to make a Strength, Dexterity, or Constitution saving throw, roll a d4 and reduce their save by the amount rolled.
  

The damage you take to gain a blood boon can’t be reduced in any way.*

* Duration
1 hour is an awkward duration in BG3. Because of how the world is miniaturised but time is not scaled, it's often equivalent to "until long rest" and Larian changed 1h durations to just that.

In D&D, 1h tends to be long enough that you can prepare when walking into a potentially dangerous area and you'll probably not have wasted it, but there's a risk you might have.

I think risk of wasting the boon is an important part of the feel of this feat. Blood magic shouldn't be safe and certain.
(Note that you may break concentration when taking self-damage, so waiting until the last moment to create a boon carries its own risk.)

My estimate of a duration with similar effect in BG3 as 1h in D&D is 10 minutes. Because I'm sure others will disagree, the mod contains two versions of the feat - one where the blood boon has a 10 minute duration and is removed on short rest (because a short rest is 1h in D&D), and one where it lasts until long rest.

* Single boon
The boon is referred to in the singular, meaning you can have one boon. Sacrificing again refreshes the duration, but does not allow you to stack multiple boons.

* Adding to attack roll
The attack roll bonus is ambiguous in when you can make the decision to use it. Is it before rolling? After rolling but before knowing the result? After you know the result? There's a big power difference between those alternatives.

"After rolling but before knowing the result" can't be implemented in BG3. The other two can, so I added both. You get an ability that can be activated to add the bonus to the next attack, and a reaction that can add the bonus after you know the result. Choose which to use depending on your interpretation.

Because of BG3 limitations, the ability version applies the bonus to the next "attack" rather than the next Attack Roll, meaning it's better than it should be for multi-hit attacks like Cleave and Eldritch Blast.

* Hit with a spell
This is also ambiguous. In D&D, "hitting" something refers specifically to attack rolls. Spells can have attack rolls so such spells would be covered without being specifically mentioned, which makes it seem like spells are mentioned in order to cover non-attack spells. But non-attack spells can't "hit" since the concept doesn't apply.

I read a non-attack spell "hit" as "dealt direct damage", so that's what I added. It's implemented as a separate reaction from the attack case so it can be toggled on its own depending on your reading of the rules.

* Damage reduction
The self-damage bypasses Resistance and Immunity, but I haven't found a way of avoiding Larian's homebrew Damage Reduction. Whether this makes you want to avoid necrotic damage reduction or stack it is up to you.



Credits
Thanks to Toarie for New Character Creation Presets WIP and Vessnelle for Vessnelle's Hair Collection, used for the character in screenshots.