The Heart of Darkness

The Heart of Darkness is Grace's unique ability.  As its name suggests, it is a heart class ability.  In the language of Nexus, Aezathol means Heart of Darkness, indicating a connection between this ability, Grace, and her dark twin Aezathol.  The heart of darkness grants Grace the ability to synchronize with the heartbeat of anything containing a piece of her body or on the verge of death to gain their seasoning and abilities, similar to how she gained some control over her yoki.  Once taken, the enemy sacrificed will die and loose all yoki if it had any.  It should be noted that this ability only allows her to take abilities and seasoning but not use them.  To use them Grace must purify/buy them back from Aezathol.  Before this happens, the stolen abilities, souls, and bodies are stored in the Void. Grace herself was never taught and never trained in this ability.  Grace herself may be able to consciously and willingly use this ability but this has yet to be revealed.  So far, Aezathol has been the one to activate her ability when the opportunity arose and has been taking her sacrifices to make Grace and herself stronger.   Use of this ability is generally painful for Grace and occurs mostly while Grace nears her limit, giving her slit silver eyes and some sort of pain, be it extreme bodily pain for normal sacrifices or a sensation of stinging hornets in her body.  The stinging comes from Grace absorbing a being with a greater quality than herself, and is a natural sensation experienced by all beings when something of higher quality is integrated into the body. The heart of darkness was created from Grace's will to save everyone (from the hate and loss she had to experience) but was corrupted by Aezathol and turned from a blessing into a curse.  
The Heart of Darkness typically is associated with silver-slit eyes and black blood when used by Grace or others who have been blessed with her ability in some way. Both of these aspects, shared with the usage of blessings by Drach, seem to suggest that perhaps the heart of darkness may be similar to a blessing for a drach. The ability itself notably is something Grace chose (as a heart class claymore) and belongs to Aezathol, who allows Grace to use it, although it was the ability she had originally.  
It should be noted that although Grace possesses a unique style of regeneration, damage rejection, it is not a part of her Heart of Darkness ability.
Grace's ability is different from other forms of integration that yoki-users possess. Impure yoki-users can devour other sources of power. Impure sources of yoki will merge together chaotically, with more dominant powers retaining more of their being. These beings can possess many skill-sets, although there is a great struggle between different yoki sources within the same body. Pure yoki-users that eat yoki-sources remain entirely themselves, wiping the ego and abilities of any yoki they have devoured. Grace can store numerous egos without influencing her mind at all and her ability allows her to freely contract and assimilate any being she encounters, adding onto her strength rapidly if she chose. She can turn these beings into blessings for herself, eliminating impurity through a difference in ego between herself and the other source of power, while simultaneously, allowing them to retain their unique abilities for her to use and learn. From these sacrifices, she can grow stronger in a wide variety of ways, rather than just a single one. However, Grace's original ability, her Heart of Darkness, can also be improved upon, but requires more work to do so. A contract must be made with someone willing to kill and sacrifice other beings for Grace, giving them a piece of Grace's ability (a skill) for them to use and season. These beings aren't immediately assimilated but instead, live their dreams out outside of Grace's existence, sacrificing to maintain their gifts. When Grace sacrifices the descendant carrying her skill, the descendant, with the seasoning it obtained, becomes the skill and a permanent part of Grace, making her stronger. In this way, Grace can grow stronger in more ways the more she sacrifices and can still improve her own original ability.
The Heart of Darkness was created from Grace's dream to save everyone, the reason she became a claymore. While this ability seems to do the exact opposite, kill everything it contacts, in truth, it does save. According to Omen, sacrificing a victim "saves" them from "hellish existence", re-uniting them with Aezathol, the goddess of the void, and eventually through Grace, allows them to exist, sometimes without memory of their lives, as pure love. Anything saved by Grace becomes truly immortal, no longer having the possibility of "dying", although they must be removed from existing in the real world, which would appear as them turning to dust and dying. Due to the apparent indiscriminate destruction that is coupled with this ability's usage, Grace fears using it, wanting to save people, not realizing that saving and destroying (who the sacrifices once were) are really the same thing and necessary for salvation, believing that her ability was a mistake.
It is likely that Blanc wants Grace to be the medium for the completion of his dream, a world without hate, where Grace would have absorbed every living being into herself to form a single collective existence of pure love. It is perhaps because of this reason, after seeing her resolve to save, that Blanc chose to allow her to become a claymore.
The Baseline Heart of Darkness Offers:
*Sucking out yoki to empower her own yoki. (This ability must be activated to absorb the observer or observed)
*Ability to absorb the mind, observer and observed of anything with yoki after weakening them sufficiently (although it is hinted that it would work on beings without yoki as well, although the benefit here would be nonexistent).  It should be noted that since Grace was unaware of her choice, Aezathol simply took the abilities for herself and gave Grace the seasoning to make her body stronger.
*Disintegration of body pieces if enough yoki is removed.
*Usage of absorbed abilities while nearing her limit by a merging of Aezathol's and Grace's abilities and seasoning (Awakening).
It should be noted that although the ability has been stated to be able to only absorb only the observer alone or the observed, the ability really absorbs both but only gives Grace and Aezathol one of the two, thus giving Grace only 1/2 of the sacrifice.
When Grace destroyed Omen by absorbing his seasoning, she gained the ability to use the offensive version of the Heart of Darkness.
The Offensive Heart of Darkness Offers:
*All the above mentioned abilities.
*Yoki Conversion - The ability to convert the seasoning and abilities of her victims into a form that she can use.  This ability allows her the option to consciously use abilities at normal levels of yoki.  
It should be noted that when Grace chooses an Observed or Observer, she is only choosing to take their seasoning to improve her body or buy new techniques (by absorbing an observed) or taking their ability and making it a part of her body without having any control over it (by absorbing an observer).  
The ability itself requires Aezathol and Grace to work together and balance. Aezathol will open up the gate to the void and presumably, Grace finds and brings the sacrifices to Aezathol, although Aezathol sometimes will take actions into her own hands and create contracts with souls that come into contact with her that eventually end with their assimilation into Grace via the Heart of Darkness. The ability always provides half of one being to both Aezathol and Grace, the piece provided being chosen upon the time of sacrifice by whichever of the two is making the sacrifice. Should another sacrifice enter accidentally through the gate as another sacrifice is made, the one who entered must be sacrificed before the gate can close. Those sacrificed by this ability (not corrupt beings) appear as they did in life, but dressed in red and black cloth. They are under Aezathol's authority and must carry out her will unless Grace chooses to use seasoning to free and forgive them. 
Grace appears to be unable to use seasoning to improve her own technique and instead, must corrupt others through contracts, offering power in exchange for the beings selling their souls to her and becoming the skills she needs. Grace herself would never do this, so Aezathol does it in her place, insisting that Grace get stronger. 
The main benefit of the Heart of Darkness is that, together with Voice, it is able to perfectly integrate the egos of other beings into Grace's own ego instead of destroying them like in aggressive melding where one mind destroys the other or regular melding which leaves a persistent, fighting mind which weakens the being. Grace can use each ability as if it were her own and not like extensions of another blessing like what occurs with drach.
For a list of individuals Grace has sacrificed, click here.Category:Terminology