How much essentia can a necrocarnate accumulate from harvesting souls?

Question

The prestige class necrocarnate at level 1 gains the supernatural ability harvest souls that, in part, says

[Y]ou can perform a short ritual to capture the soul energy of a newly dead corpse. This ritual requires 1 minute of uninterrupted concentration plus the corpse of a living creature that has been dead for no longer than 1 hour per necrocarnate level you possess. At the end of the ritual, you gain a number of essentia points equal to one-half your necrocarnate level (rounded up). This benefit lasts for 24 hours. No corpse can be used for this purpose more than once. (Magic of Incarnum 132)

Is the essentia gained from the supernatural ability harvest souls cumulative? For example, does a level 1 necrocarnate that uses the ability harvest souls twice on two recently dead corpses in as many minutes gain 2 essentia, 1 of which, after 1 day, will be gone 1 min. later than other?

Or, instead,—much like temporary hp from the same source,—does the next application of the supernatural ability harvest souls in the same 24-hour period replace the previous application of the ability harvest souls, updating to their new values the amount of harvested essentia the necrocarnate gains and the duration the necrocarnate's harvested essentia perists? For example, does a level 1 necrocarnate that uses the ability harvest souls twice on two recently dead corpses in as many minutes gain but 1 essentia, the second corpse's essentia replacing the first corpse's essentia and the second corpse's essentia fading 1 day after its acquisition?

Or is there another different way to read this special ability that I'm overlooking?

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Note: While Jalas remains 4 levels away from his first level of necrocarnate, his player is considering next level an improved familiar that will leave Jalas unable to enter the prestige class necrocarnate when Jalas is level 8. The DM already knows that, yes, if the effects of the ability harvest souls is not cumulative only one level in necrocarnate for the special ability harvest souls isn't worth it for a meldshaper, but the ability harvest souls becomes staggeringly good if its effects are cumulative… and even worth delaying the acquisition of a glacier snake improved familiar despite the glacier snake's awesome Serpent Kingdoms description: "A glacier snake’s 12-foot body is covered with a thick, brown pelt[;] razor-sharp teeth surround its mouth in a ring, and it has a bony stinger at the tip of its tail" (84). Yes, Jalas's choice is between harvesting the souls of the dead sooner and having a 12-ft.-long furry snake later or vice versa. And, yes, Jalas does look like he's stepped out of an 80s heavy metal album cover.

Answer 1

Raw is ambiguous

As far as I can tell, stacking is a matter of how you interpret the stacking-rules applying to Essentia. The bonus is never typed, and unlike temp-HP, bonus essentia from multiple sources stack.

The problem here is that this bonus essentia is from the same "source", and that ambiguity, to my knowledge, is never covered in the MoI ruleset.

Divining the ruling from the rules given

1. The ability's action cost is 1 minute at first, but at level 10, essentially as part of your capstone, the ability is dropped to a full-round action. If this ability doesn't self-stack, the action-cost, either way, is almost entirely irrelevant, as its exceedingly rare for a PC to go long periods of time without a rest suitable for performing this ritual. For that bump to be notable*, the ability needs to self-stack.

2. Necrocarnates have no essentia progress innately, and are, imho, meant to be at least competitive with Incarnates, except they're kick-the-puppy evil, which means they're probably meant too outstrip an Incarnate, and be used as a boss-type class. To that point, balance-wise, we have two clear options: Non-stacking makes Necrocarnate weaker than straight Incarnate. Stacking makes them stronger than Incarnate.

*The bump doesn't really have to be notable, though. However, if its not a notable bump, this poster is left asking why they have the action-cost change at all, instead of just giving the better functionality at level 1.

Balance

While Essentia is REALLY nice to have, in my experience, the more you have, the less valuable each additional point is. I'm personally unaware of any way to even spend "infinite" Essentia, such that this ability even could get out of hand.

Answer 2

Necrocarnate is a mess, and I don’t recommend using it. This homebrew rewrite is by an author I trust, and has good reviews (but I have not played it or seen it played myself).

That said, ultimately, I must come down on the side of saying it stacks. Quite simply, nothing says it doesn’t, and the choice to limit harvest souls to half the necrocarnate’s level, a lot less than any meldshaper would gain naturally with the same levels, suggests that the intent was for it to be used repeatedly (two uses in a day gets you back where you should be, three or more is the point where it is a benefit).

The obvious problem with this kind of thing is one colloquially known as the “bag of rats” problem, and it comes up with a lot of necromancy effects. Harvest souls, consumptive field, and the like are all based on “get some bonus that grows the more you kill stuff” which is clearly thematic, but really problematic when it comes to cases where a character can trivially kill lots of stuff, say, by dumping a bag of rats on the ground and setting it on fire, or pouring burning oil down an anthill, or whatever.

However, the existence of this problem reinforces the suggestion, to me, that harvest souls was intended to stack with itself—Wizards of the Coast has done the same thing elsewhere, with the same potential problem, and never addressed that problem or what to do about it. They apparently either felt it wasn’t a problem, or it was a problem that DMs should address themselves, without the need for any guidance from Wizards of the Coast. I find that perplexing, though, since I myself have difficulty determining exactly how one should handle harvest souls that is fair.

Certainly, trivially gaining “all the essentia” is a problem. While essentia does not have endless benefits, so there are limits on how much you can do with it, “just” filling every essentia receptacle you have is already very substantial—after all, that is the incarnate’s capstone perfect meldshaper class feature. So harvest souls either needs to be limited, or suitably difficult to use, or both, to be worthwhile.

The choice by sirpercival, who wrote the homebrew I linked above, is to have it become the effect of a damaging touch attack, usable at will, but only last a few rounds and only be usable to fill necrocarnum soulmelds. Certainly workable, since his necrocarnate is also progressing the character’s base class essentia on 10/13 levels, but too weak to be just stapled on to the non-progressing necrocarnate in Magic of Incarnum.

Keeping it more similar to the harvest souls ability found in Magic of Incarnum, I would lean towards tying the essentia you gain to the HD of the creature slain, to a max of half your necrocarnate level. So, perhaps,

At the end of the ritual, you gain a number of essentia points equal to one-quarter the corpse’s HD in life (to a maximum of one-half your necrocarnate level, rounded up). This benefit lasts for 24 hours. No corpse can be used for this purpose more than once, but the benefits of multiple rituals using multiple corpses stack.

I would probably also allow a single ritual to cover multiple corpses at once, possibly extending the time proportionally, to allow for it to grant one-quarter of the sum total of their HD (still to a maximum of one-half the class level rounded up).

The effect of this is to allow a necrocarnate to build up his essentia, but only with creatures of some HD. A “bag of heavy warhorses” simply seems a bit more tricky to work with than a “bag of rats.”

The problem with this approach, however, is that until the necrocarnate gets some kills under his belt, he is lagging way behind other meldshapers—which limits his ability to kill things. It makes actually playing a necrocarnate very awkward and difficult, and may not be best for your game. Which is why sirpercival changed the necrocarnate to mostly progress meldshaping normally, and to have harvesting souls only offer a small amount of temporary benefit.