@Assalt Tesla is a really fragile existence. To understand the overall scope of this story you have to read multiple series, gahkthun alone doesn't do it.
Basically in the past there was a country, one day the members of the country saw light and decided to leave their king alone in that country, said king hasn't moved from his throne in thousands of years and as a result has lost emotions, now his country is dark precisely because he feels nothing.
That country was
M (Shikkoku no Sharnoth) country, it exists in another dimension, the members of the country are Old Ones, they are conceptual beings, the light they saw was humans so they went to earth.
One of the Old Ones, the thunderbird, met tesla and he became cursed, as a result Tesla doesn't age, and is undying. From the moment he came into contact with the Thunderbird, something that couldn't possibly exist, he has existed outside the bounds of the world. The world itself rejects him as his powers are incompatible with the laws of the world, as such humans can't remember him, as he isn't even supposed to exist. He is incapable of existing even in memory, and when everyone in the world forgets about him, he will lose his powers and die.
In other words he needs people to remember him to continue existing, well see GK planned to kill tesla so he gathered everyone related to him to the island where the story takes place, and was going to destroy the island with everyone on it, with no one remembering tesla he fades out of existence.
Neon, female protag of gahkthun, her family is special in that they can remember tesla when ordinary he fades from ppl's memory in like a year or something.
Wilhelm a member of the council found out that neon was tesla's weakness and separated them, as such tesla became weak, because he feeds off neon's radiance.
In short the tesla who fought gk was heavily weakened, and this isn't even taken into account he was already weak at the beginning of the story because he was suffering from the effects of being sealed in the time prison.
