Dagon had coveted the human vessel the Abyssal Sovereign had left behind in the material world for a long, long time.
In fact, Dagon wasn't the only one coveting this "divine shell." According to Triss and Lady Dena, every kind of spirit and evil being in the material world that knew the Abyssal Sovereign's story and possessed the ability to "possess" others regarded the Sovereign's human vessel as a treasure that could let them "ascend in a single step."
That wasn't entirely wrong.
After all, the Abyssal Sovereign's consciousness had left this vessel and returned to the subspace centuries ago, following the great war. Its shell remained behind as the strongest thing in the world. In theory, anyone who could implant their soul and consciousness into it could wield the genuine power of a god.
Unfortunately, the divine domain surrounding the human vessel, which kept "mortals at a distance," was enough on its own to dispel the bold ambitions of 99% of would-be usurpers.
Only beings of the same level could corrupt and control this vessel. And, as it happened, Dagon was the one most qualified to enjoy this "feast." But the Fish Father had reasons of its own for obsessing over the Sovereign's human vessel.
The Father of Fish only looked like a fishman. It wasn't actually a fishman that had ascended to godhood; its true body was a created symbiote.
No matter how powerful a thing like that was, it needed a host to unleash its full potential. Based on Murphy and the others' earlier survey of the coastal observation zone near the Land of Origin, Dagon's current body was probably the result of it slipping into the material world during the Chaos invasion and corrupting "Poseidon," the biological weapon Eden District had stationed to protect the Tidal