How much HP is a 5th level warlock spell worth? - Balancing a warlock invocation

[tyriuth]

I'm making some invocations to go with a pact to fit in with a custom setting.

Currently the invocation reads:

Bloodstone
Prerequisite: Level 12, Pact of the Stone feature,
As a bonus action you can sacrifice some of your life essence to create additional spell slots.
Using this ability deals damage to you equal to 1/3 of your HD rounded up.
This damage can't be prevented or reduced in any way.

For example, a 15th level Warlock using this ability would take 5d8 damage, and a 17th level Warlock would take 6d8 damage.

I was hoping to make it an ability you wouldn't want to use more than once an encounter without good reason, without specifically limiting it. So I ask, does this seem like a reasonable price to pay, should I increase/decrease the damage, or is this going to be a nightmare to balance?

[zeek0]

What if you simply took away the Hit Dice directly? This would make the trade less immediately dangerous, but you'd still miss out on HP at some point in the future. Perhaps 1/2 is a better number for this option, however?

[tyriuth]

So just deal 1/3 or 1/2 damage? It doesn't seem like a good tradeoff for one spell slot is all. Unless I'm missing something game-breaking about the spell selection that warlocks have?

[zeek0]

I mean, just reduce your Hit Dice directly. You don't take damage now - you just can't heal yourself properly later.

Warlocks are balanced around having only 2-4 spells per short rest. To mess with that balance is powerful, and I'm just being careful.

[tyriuth]

That's fair, I might just scrap the idea and think of something else :\

[ravencroft0]

Your invocation description doesn't even say how many spell slots you get. Can't really comment on balance if we don't see the actual gain. Some ideas though: the amount of hd rolled could instead affect your maximum hit points. You don't regain the use of those hit points until you have used the gained spell slot. You do not immediately gain those lost hit points. You will require the aid of healing spells or normal healing methods.

The concept of the invocation isn't bad, but you're leaving out vital info.

[Rynjin]

I would say for damage dealt, you balance point should likely be at least Warlock level plus a reasonable Cha mod (3-4). Reason being that any less damage than that is basically free spell slots at-will for an Infernal Pact Warlock. At the level you can get it, you want it to deal at least 16 damage, for instance.

By your math, a 4d8 at level 16 seems kinda fair, but also doesn't take into account party healing also making the tradeoff easily recovered.

Reducing their HD usable for recovery does sound like an interesting trade. Limit it based on that to how many uses you want it to have total (1/3 HD is 3/day, 1/2 HD is 2/day, etc.).