Deep-Sea Survival for All: I Summon Cthulhu from the Start

[Light Lovecraftian elements + a world with light puzzle-solving + decisive and ruthless + not an invincible protagonist + gradually goes completely off the rails in the middle and later chapters!] Yan Zai’an is mentally ill. At least, that’s what people said before the sea swallowed the world. Later, everyone really did plunge into the deep sea, just as he’d predicted. [Deep Blue System warning! Bladder oxygen level dropping rapidly!] [Warning! Unknown influence detected. SAN value dropping rapidly!] [Lost city discovered!] [Warning! Current depth: -12,800 meters. The third wave of geothermal heat is closing in!] In an instant, people already buckling under unimaginable pressure begin to struggle frantically, rising toward the hopeful sea surface and the homeland they left behind, fighting to survive. But ten thousand meters down in the abyssal sea, Yan Zai’an, piloting a mini-submarine, with twisted tentacles growing from his back, constantly hunting and uncovering more of his own curse, finally understands: He had already become something monstrous...