An Abyssal Exalted portraits the fallen hero, a former Solar Exalted who, for some reason or another, turned away from the light and forsake his divine conception, becoming the antithesis of what he should be. Redemption is, at first, not a possibility for him.
However, that is a lie. Solar shards were created by the gods with the help of Autochthon, the machine god. Therefore, they are the only ones who are able to permanently alter the shards, no one else. Redemption of Abyssal shards is a possibility, and a source of concern to deathlords.
The Monstrance of Celestial Portion
The Monstrance is a cage that deathlords use to corrupt Solar Essences, converting them into Abyssal ones. It is made of bones and black jade, and runes are carved in its edges, making it immune to both Emerald and Sapphire circle sorcery. It is in nature offensive to the core of every solar, their power being so great and evil that the mere sight of a monstrance disturbs a solar in a way no other artifact in Creation is able to.
Its construction is a big mystery, known only to the Deathlords and a few very trustworthy servants. The design that the Yozis gave to the deathlords was based on original work by a Solar Exalted from the First Age, whose name is unknown. It is known, however, that he was a highly skilled twilight sorcerer. He designed the original monstrance because he didn't want Lytek to put his hands on his shard. He believed that it would be a mutilation of his soul and therefore it had to be avoided. He designed the monstrance so he would be able to choose, by himself, the next host for his shard.
When the time for his death came near, he would go and find a mortal boy with the potential to be a great sorcerer. Then he would take the boy into his protection and tutor him in the basics of occult and sorcery, as well as other necessary skills for a sorcerer. The boy would become a savant in only a few years, not only due to his potential but also to the exceptional teaching skills of the solar. He would then ritualistically imprison his pupil into the monstrance and kill himself, in a way to lure his shard towards the monstrance and into the boy, exalting him.
The Yozis stole the knowledge about the original monstrance and altered it, though the concept is basically the same. The Yozi version uses the ritual to extract a portion of the shard and link it to the cage, while a powerful spell cast by the deathlord infuses the shard with the spellcaster's own essence. They have also altered the runes inscripted in the cage accordingly.
Corrupting Solar Shards
Deathlords use torment in a Monstrance to break the will of a solar exalted, luring him into giving away his name to the void.
Once the solar has accepted that death is larger than life, even if only for a moment, he is lost. He leaves the monstrance and reenters it, this time willingly. The deathlord then invokes the Dimming of the Light spell (Ab:229), which extracts a portion of the shard, linking it to the monstrance, and filling the space left by that portion with the deathlord's own dark essence. While inside the monstrance, the solar gives up every power he has and, lastly, his name. Giving up his name concretizes the extraction of such portion of the shard, linking it to the monstrance (the name is forever lost, although). The deathlord then infuses the solar with his own dark essence, filling the empty left by the portion of the shard which was linked to the cage.
In nature, Abyssal shards are mostly identical to Solar shards - the difference is a small part of it that has been replaced by the deathlord's own essence. That makes a Deathlord's deathknights the carriers of a portion of their own master's soul. A deathlord cannot be completely destroyed without his deathknights being defeated - in a way, they act as phylacteries, or in more "trendy" terms, horcruxes.
Recovering Abyssal Shards
There are two ways for an Abyssal shard to become one again and retake its original Solar form. The first one is quite troublesome, since it requires the Abyssal to wish for redemption with his heart, discover the path to redemption by himself and get an advocate to intercede for him with the Unconquered Sun himself. In case of success, the missing portion of the shard is reintegrated into the shard through the cleansing of the dark essence in its place. This is not done at no cost for the Unconquered Sun, though: he absorbs the tainted portion into himself, replacing it with his own essence. The tainted portion remains with him, being gradually restored.
The other way to do it involves a more direct approach - though it's pretty dangerous, too.
Basically speaking, un-tainting an Abyssal shard requires the lost portion of the shard to be taken off from the monstrance and reintegrated into the original shard. For that to be done, direct access to both shard and monstrance is required. This is the dangerous part of the process: both abyssal and his monstrance need to be taken from the deathlord, who is unlikely to give them away willingly. The Abyssal shard, then, must be cleaned and stripped off the dark essence of the deathlord. The portion in the monstrance then must be liberated and reforged into the shard, turning it back into a Solar one. However, the life of the Abyssal is lost during the process, and his higher soul reenters the natural cycle. Upon reincarnation, the reborn Solar shard is granted a new name and the process is complete, although memories from existence as an Abyssal may return to haunt him.
During the process, the portion of the Deathlord's soul is destroyed or, if not taken care of, becomes a very angry and quite powerful hungry ghost - a reflection of the Deathlord when he cast the spell that tainted that essence.
The Great Curse
Neither of the processes that recover an Abyssal shard affect the Great Curse. The taint is laid upon the shard by the deathlords, who ultimately are former solar exalted. Therefore, it can be undone by someone on the same level. The Curse, on the other hand, was laid upon them by the dying Primordials - to lift its mark from the shard requires, at least, the same power level. That means that, basically, there is no power in or out of Creation capable of completely erasing the Great Curse from the Exalted. Ressonance in the Abyssals is just another manifestation of the Curse itself, triggered by the deathlord's essence in the shard.
Upon cleansing of the shard, the Curse retakes its original form, preying on the new host's virtues as usual.
Comments and contributions welcome. --Nabeshin