Cursed Arm: Impressive. So your previous body was not your own, either?
Zouken: …Oh? Were you watching, Assassin? Yes, my body has perished a long time ago. I am just an old fool who has to leech off of a whole body. My body cannot handle walking under the sun like everyone else's.
The laughing figure is indeed Matou Zouken. It took him a few minutes to attack and take over a woman's body. The speed is incredible, even for a master magus.
Cursed Arm: …I see. So you did not care who it was. All you require is a full body, no matter whose it may be. You merely mold the body to the shape you want. What's inside your body are worms in any case. The worms will function as your organs… It is indeed a mimicry.
Zouken: Oh my. In only one day, you have grown quite talkative. Do you remember your wish now?
Cursed Arm: Of course. I can only be summoned by people who share my desire. Your craving for eternal life has summoned me here. What I wish for is eternity. But this raises a question. You are already immortal. You have lived for over five centuries. Then, is your wish not already granted?
Zouken: Hah
The old magus lets out a cry. Who would know that the cry is not a laugh, but a cry of anger? Yes, Matou Zouken is almost immortal. He will live as long as there are bodies to infest and his soul remains alive. But there is only pain. Matou Zouken does not wear his current form by choice. Turning his body into something inhuman and mimicking human form. There is a limit to this magic. It is like a machine. A machine can run forever in theory, but its parts rust over time. Parts rust. Cogwheels slide out of place. Programs become obsolete.
Zouken: No, no. I am not immortal, Assassin. My body rots. It will rot no matter how many times I get a new body. It will rot no matter what I do. This new body has begun to rot already. Can you begin to understand the discomfort and disgrace of rotting alive, the despair upon learning that your body is not human?
Cursed Arm: Why does it rot? Are the worms not able to make a proper body?
Zouken: Haha, that is not true. The worms are lively. They have no problem as a body. From the start, I became a parasite in grief over my aging body. They serve perfectly as a body. I chose this method to prolong my life forever while retaining my humility.
Cursed Arm: I am even more confused. Is that all you can manage, even with the power to perfectly preserve your body? First of all, why are you taking the form of an old man? If you are able to shape the flesh into any form, you should be able to take any appearance.
Zouken: That is a good question. Then let me ask you. Let's say something that lost its original body tries to reproduce its body with its powers. In that case, what do you think restores the body to its original form?
Cursed Arm: The body records itself. Even after it is burned or cut, the body is able to restore its original appearance, because there is an internal blueprint.
Zouken: Yes. A body's composition is recorded in its genes. But it is different in my case. I lost my body, the place where my composition was recorded. I can no longer use that blueprint to recreate my body. In that case, what do you think records my body and gives it form?
It is obvious. The soul. It is not the law of the physical world, but something of the higher order, the record of all bodies within the concept of the astral world. Yes, if one has the soul, one should be able to reconstruct their body even if the original body and genes have been lost. So, in other words, this old magus is...
Cursed Arm: I see. You keep your soul alive, while taking the bodies of other creatures as your own. So that is the trick behind your immortality. That must be why you cannot turn yourself into anything else. What you are keeping alive is your soul, not your body. So that is why you cannot take any other form but that of Matou Zouken?
Zouken: Of course. I am not taking form as an old man because I want to. Look, this is the only form I can create. And it is a poor creation that rots away unless I exchange it periodically. I used to be able to last fifty years before I had to exchange my body, but I have to exchange it every few months now. Who can understand the pain I feel as I rot away? Do not misunderstand, Assassin of this generation. Do not call such a thing immortality.
Irritation can be heard in the voice of the old magus. It has been three days since Assassin's summoning. He finally sees the true nature of his summoner.
Cursed Arm: I understand now. So what is rotting is not your body, but…
Zouken: Yes, my soul is rotting. Time even affects ethereal bodies. Therefore, my physical body will rot. It is natural for my body to rot if my soul, the blueprint of my body, is rotten.