The Corpse King is a recreation of the mind of Artoria Pendragon created using Logos React, similar to how Add's personality is a copy of Kay. She was created for the purpose of reviving Artoria in modern times, by combining the Corpse King as the mind, Gray as the body, and Saber summoned in the Holy Grail War as the soul.
The Corpse King dwells in the secret tunnels beneath the Blackmore Cemetery. Ten years ago she awoke, at the same time that Gray's face changed to resemble Artoria's. Only Grandmother, the leader of the village, knows of the Corpse King's existence, though the Holy Church has their suspicions.
Though she is a recreation of Artoria's mind, she does not possess all of Artoria's memories. Accessing memories is the purpose of the brain, which is located in the body. For instance she did not recognize Kay.
Inside Logos React's recreation the Corpse King is subtly influenced by Magdalena, but she herself didn't even realize Magdalena was there. She wanted Gray to flee the village and never return, but once Gray came back the Corpse King had no choice but to detain her.
Originally the Corpse King did not have a face. The mind of King Arthur could not materialize completely, much like how when Kay manifests from Add he appears foggy and only halfway there. She was given a physical form by inhabiting the body of Magdalena, Gray's mother. She only possesses this body in the simulation inside Logos React.
Lord El-Melloi II Case Files
Two months before the start of the series the Corpse King was killed. Gray's mother Magdalena, after spending years synchronizing herself to both Gray and Artoria's existences, stabbed herself and then used Erosion to cut out her mind and soul so that the Corpse King entered her dying body, killing them both to save Gray.
Gray, who didn't know who the Corpse King was at the time, was greatly disturbed seeing the corpse of someone identical to her suddenly appear. After her death, Bersac Blackmore told Lord El-Melloi II to take Gray to London and never return.
case. Atlas Contract
When they entered a simulation of Blackmore Cemetery from several months ago inside of Logos React, Lord El-Melloi II and Gray ventured into the tunnels beneath the village via the church where they encountered the Corpse King. The Corpse King told Gray and Waver that it was too early and that the future king hasn't awoken, and that Gray was supposed to be on the surface, whereas she was meant to be underground.
Later the Corpse King attacked Fernando Croze and Sister Ilumia in the tunnels, then engaged with Kay when he, El-Melloi II, and Gray got involved. When Gray spoke to the Corpse King about her experiences beyond the village, the Corpse King realized that they were inside a Logos React simulation of the past instead of the real world. Deciding that therefore none of it mattered, she began to use her Rhongomyniad on the assembled group. A sudden flood from the swamp above the tunnels allowed everyone to escape.
When the temple in the tunnels rose to the surface and appeared in the swamp, the Corpse King's corpse was with it. Lord El-Melloi II unmasked her, revealing Magdalena's face beneath, and the second, separate copy of Magdalena that the simulation had created disappeared.
After the Corpse King's death in the simulation, its mind "rebooted" and it revealed itself as the very avatar of Logos React, the Atlas Mystic Code's personification inside the simulation. Because Logos React is an inanimate object it cannot die, but its avatar the Corpse King died and sent the information of "death" into Logos React's systems, causing a paradox. This paradox caused Logos React to malfunction, and as one of Atlas's Superweapons its malfunction was a threat to humanity that threatened to destroy at least Blackmore Cemetery, and potentially half of Wales.
Gray and her friends confronted the avatar of Logos React, which summoned crystal soldiers and simulations of Sister Ilumia and Bersac Blackmore to fight by its side. Gray battled its dark Rhongomyniad not with Rhongomyniad of her own, but simply with Add in his scythe form. After gaining the advantage, Gray plunged the soul-cutting dagger Erosion into the avatar to cut out the dead mind of the Corpse King from Logos React.
However, Logos React held onto its avatar and prevented the splitting in order to further understand the concept of "death". After being stabbed the avatar and all the simulated soldiers it had summoned turned into the red sand of Atlas's Philosopher's Stones, which began to expand until the entire simulated world became a red desert. Logos React's avatar took the form of a massive bird comprised of the red sand, representing the bird of HermesWP. Recognizing Gray as its death, the avatar decided to kill her.
Kay held off the bird's attack with his Noble Phantasm, Camelot Image, and then sent the attack back at it. Kay's unstable Spirit Origin was destroyed by the act because he was not fully manifested, but once he disappeared Add woke back up, allowing Gray to hit the avatar with all of her power. Using Add's boomerang form as a hang glider, Gray climbed atop the bird's back, and fighting off its attacks with Add's scythe form, stabbed Erosion into the avatar and then used Add's hammer form to plunge it in. With Flat hacking Logos React and Svin providing him magical energy, the two bound the collapsing bird in chains.
With the avatar's defeat, the mind of King Arthur was returned into Logos React as a mere mental model. Logos React entered a state of self-diagnosis and repair, and would remain that way for a few years at least. As per their contract with Atlas, Blackmore Cemetery would continue to hold on to Logos React until either King Arthur was revived or was deemed impossible to revive.
The Corpse King has the ability to give life to an entire army of skeletal soldiers, albeit barely functioning. Magical energy taken from the surroundings are inserted into records and use skeletons as a medium. In a sense, they're almost like failed Servants. The skeletons are powered by od from the villagers of Blackmore Cemetery. As the villagers pray to the Black Madonna statue in the village's church, it is equivalent to offering up their own magical energy. As a result, the Corpse King's skeleton soldiers can regenerate indefinitely, regrowing broken parts like a video being played in reverse. Even so, they can be defeated by the Baptism Sacrament of a priest of the Holy Church.
The Corpse King wields a dark Rhongomyniad. As Gray is the body of King Arthur and the Corpse King the mind, it only makes sense that they would have similar lances. Hers and Gray's lances are only shadows of the true Tower of the End, so it is not impossible for multiple shadows to exist. The dark Rhongomyniad is covered in thorns resembling wolf fangs. Rather than light, its magical energy manifests as a whirling cyclone of wind. In other words, it is similar to Artoria Lancer Alter's version.
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 1 Part 4
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 2 Part 4
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 4 Part 2
- Lord El-Melloi II Case Files Volume 6: case. Atlas Contract (Upper) - Chapter 4 Part 4
- Lord El-Melloi II Case Files Volume 6: case. Atlas Contract (Upper)
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 1 Part 3
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 2 Part 5
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 5 Part 1
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 5 Part 3
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 5 Part 4
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 5 Part 5
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Epilogue
- Lord El-Melloi II Case Files Volume 7: case. Atlas Contract (Lower) - Chapter 2 Part 3