What if the black king was Zeus's father Kronos?! If that's true this could be mind blowing!
Sorryz meant to like!
Well, one of reason why, was that Greek mythology have so many sources that there's no definite/constant story about them.
If you mean how Zeus came to power, the most well-known answer is through rebellion together with all his 5 other siblings, decapitating his Titan father 'Cronus' body, mutilate his body. Afterwards, He, Poseidon, and Hades got into infighting on deciding who get what, and by 'deceiving' the two, He won authority over the sky.
Thus, there's also that possibility of Cronus grudge over the Olympian gods.
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But, I think there's something more mysterious in Black King and MC's fate.
I mean, black is a color and also some kind of aspect, but black doesn't really mean death....
Mr.Hermes is a cute lad, but somehow his helmet didn't match his appearance...
It is still a mystery of why MC obtained 2 god-class items with completely opposite nature.
If he's a successor to Hades, it should be 'Underworld God' rather than 'Black King'.
But, If he could utilize primal chaos energy, the problem of two different nature are solvable tho...