Heck, Heart-Eating fist is basically just a fancy way to say "have enough damage to kill an enemy with double 10s". Because if you can roll successes equal to their health levels - then you would kill them without that charm. Rolling 10 successes to kill someone with 10 remaining HLs takes at least 20 Initiative, which (with double 10s) would give you a decisive attack with 10 damage anyway.
So what you're paying for here is the effective double-10s (which you can gain in other ways), a bypass of emergency damage reduction charms and hardness (you're not technically killed by damage, so charms that reduce decisive damage won't take effect) and mostly the effect from consuming the enemies heart.
And remember, even killing a mortal with this is a difficulty 7 gambit for which you should have 14+ Initiative.
With that 14 Initiative, Pattern-Spider Touch can easily execute a starting Solar (pay 7i, then better than even odds of success with the remaining 7i). Which also bypasses damage reducers, but also Ox-Bodies. And can turn anyone who doesn't have certain Integrity-charms into an ally, can be used without killing anyone (a huge advantage! you don't always want to kill) and generally has more applications.
Again, what I would do with Pattern-Spider Touch would be something like this:
Pattern-Spider Touch
Cost: ???
Type: Simple
Keywords: Decisive-only
Duration: Instant
The Martial Artist may strike at their enemies very fate, poisoning it like one of the Pattern-Spiders that weave the world of the fate might. This is a unarmed decisive attack that deals bashing, lethal or aggravated damage, at her choice. She may also choose not to deal any damage, in this case the effect of this charm is calculated but the enemy does not actually lose any health levels.
A strike at her opponents very being such as this is opposed by their vitality. The effects of this charm vary depending on how many health levels the enemy loses from this attack, and the wound penalty of these health levels:
- If the enemy loses a -0 health level, the martial artist can change their connection to others. For every health level lost by the opponent, she makes one reflexive Instill action that automatically overcomes their resolve. The enemy can spend willpower as normal to resist this, and this can not effect an enemy with no willpower left. She can make reflexive Read Intentions action in place of a Instill action, but this does not bypass the enemies guile.
- If the enemy loses a -1 health level, the martial artist can alter their standing in the world. For every health level lost by the enemy, she can make the enemy lose a dot from a story merit (Allies, Backing, Command, Contacts, Cult, Followers, Influence, Mentor, Retainer, Resources, but not Artifact, Demense, Familiar or Manse). The enemy can spend willpower for each dot they would lose instead. Alternatively, the martial artist can grant background dots to the enemy, but such connections erode at a rate of 1 dot per week (down to their original rating) and the martial artist can not use this effect on herself.
- If the enemy loses a -2 health level, the martial artist can warp their very being. She can inflict the enemy with any disease of her choosing, or with a poison. This poison deals damage equal to her Essence (the damage can be bashing or lethal), can have any interval between minutes and weeks as she chooses, has a penalty of -3 and lasts for as many health levels as her enemy has lost. Alternatively, she can warp her opponents mind or body. This works like a crippling injury (of any severity she chooses), but can also inflict a derangement.
- If the enemy loses a -4 health level, the martial artist can strike at their soul. The enemy loses one willpower per lost health level, which occurs before this charms other effects. If the enemy would lose more willpower than they have, the martial artist can deny them access to one of their charms for each excess point. Each day, the enemy can spend one willpower to regain access to one charm, until they have recovered all their charms. If the enemy runs out of willpower from this effect, the martial artist can still make up to ten instill-actions on the enemy.
- If the enemy loses their incapacitated health level, the martial artist can erase them from existence. The target dies and leaves no body behind, while their belongings are scattered to a nearby location where they may belong (clothe might appear folded in some closet, artifacts will be buried in appropriate tomb etc.). This shattering of fate also ripples through the memories of anyone nearby - the martial artist rolls (Charisma + Presence) with a target number of 4 and (targets Essence) automatic successes. Anyone within extreme range whose Resolve (modified by any Intimacies towards the target) is overcome loses all memories of the target and can not notice any inconsistency this creates in their memories. Anyone who has lost their memory can regain it by spending one willpower per week for (martial artists Essence) weeks.
All of the effects, other than the damage, are treated as curse-effects for the purpose of charms that defend against them.
(Some sort of rule for using this outside of combat, not sure how to do it yet).
Wording needs work, and some of those effects may or may not be too powerful.
One possible change would be to make each effect "for each -0 (-1/-2/-4) health level lost", instead of "for each health level lost. This would weaken each effect quite a bit, possibly too much in some cases.
The charm might also get a limit on uses, maybe just once per scene or something like that. And of course I haven't decided on cost yet.
If you hit with 20 Initiative, you'd be able to inflict 10 automatic instill-actions on the enemy. That's very powerful, enough to make or destroy three defining Intimacies. However, the target can spend Willpower to prevent this as normal. To be safe against complete re-writing, I put a cap on the amount of damage you can do on this effect.
Against a lot of enemies, you'll quickly come into the -1 territory. At that point, you can do more permanent damage, though Exalts will be able to defend against this with curse-resisting charms, and Willpower is still in play.
To get at -2 health levels, you have to do four damage on mortal, and against a Solar with two Ox-Bodies it'd be six damage (Iron Skin Concentration quickly raises this to nine). At that point, you're inflicting a -3 poison penalty or doubling their wound penalties, both of which can be resisted.
To get at -4 health levels, you'd need six damage on a mortal, ten against our Solar (or easily more). This level basically guarantees that previous effects will happen, and of course draining that much willpower is pretty strong on it's own too.
The -incap effect allows a Sidereal to kill someone, even in a dramatic duel, without anyone noticing. Well, anyone but the strongest Exalts that is. It's appropriately scary, but doesn't actually make it easier to kill an enemy.