Sangromancy (Blood Magic) warlock patron feature

Question

Considering this ability for a homebrew warlock patron, and want to know if the wording works, on a technical level as well as a readability level, and if the overall effect is balanced. Obviously, this is only one feature of the patron, so a comparison against other 6th-level features of warlock patrons will have to do.

Sangromancy

Starting at 6th level, spells from your invocations can be cast at will, without expending spell slots. However, if you would otherwise need a spell slot or rest to cast it, you can only cast it using blood magic. When you cast a spell with blood magic, you must spend a number of Hit Dice equal to half the spell’s level, rounded up, rolling them and taking that much damage. (You do not add any bonus to this roll.)

For example, after casting animate dead from the undying servitude invocation, you may cast it again before completing a long rest by spending 2 HD and taking damage equal to the amount rolled on them.

For more examples beyond what I intend to put in the text,

• Mage armor from armor of shadows would never need blood magic, because it never requires rest or a spell slot.

• One could cast the compulsion spell of bewitching whispers without any spell slot, and could do so more than once per long rest, by spending 2 HD and taking the damage rolled on them.

Answer 1

It is hard to judge in a vacuum, since the other pact boons might make it better or worse.

Compared to the 6th level pact powers, it seems costly and conditional.

Misty Escape, Dark One's Own Luck, and Entropic Ward can be used once per short or long rest, without any other cost. They do not require you to take any specific invocations.

Radiant Soul and Elemental Gift both give the warlock a resistance. Radiant Soul also increases damage for certain spells, and elemental gift gives you flight 2-6 times per long rest; it's not as good as the fly spell, but doesn't require concentration.

Guardian Coil lets the warlock reduce damage with a reaction, while they have a tentacle active and the character taking damage is within ten feet. It is a bit more difficult to judge. But Fathomless also grants oceanic soul at level 6, which also gives a resistance, and effectively grants tongues while submerged.

Accursed Soul is similar to the lower level conjure spells, except it is cast by killing an enemy, doesn't take concentration, and is once per long rest. Again, somewhat more difficult to compare.

Sangromancy does damage to the warlock, and reduces their Hit Dice pool. This is a double hit, the warlock both takes damage and reduces their ability to recover during a short rest. It is the only pact boon at level six with an additional cost, outside of guardian coil requiring a tentacle be summoned. And the tentacle can also attack, so arguably it is less of a cost than self injury.

In addition, sangromancy only works if the warlock takes an invocation that allows them to cast a spell using a warlock spell slot. And there are only 8 of them (between the PHB, TCoE and XGtE the books I have on hand), versus 15 that are already 'cast without a spell slot'. And that is ignoring the other sorts of invocations (EB invocations, skill invocations, passive invocations, etc.).

The main advantage is that all of those invocations can only be cast once between long rests, and sangromancy allows them to be used multiple times. However most of the spells/invocations in this category are concentration spells, which also makes this ability less effective (undying servitude is the only one I see that isn't concentration). If the warlock is concentrating, they (generally) can't use another one of these spells. And if they lose concentration due to taking damage, the loss of HP to recast the spell becomes more dangerous and untenable. And since warlock spells slots are recovered after a short rest, unless you specifically need to cast the same invocation spell multiple times between a long rest, it might be better to just use the slot and use a different invocation.

Also, what about trickster's escape and gift of the depths, which do not require a spell slot, but are also once per long rest? I assumed they were not effected and counted them in the no slot category, but an argument could be made either way.

If the patron and other benefits lend themselves to a more casting oriented warlock, with reasons to take those sorts of incantations, then sangromancy might be closer to being balanced. But as it stands, it doesn't feel balanced compared to other level 6 patron boons.

My suggestion would be to make the cost a flat 2 HP per spell level and 1 HD. And consider a second, minor sixth level boon. Perhaps advantage on all saves versus frightened?