Y'know the gist: But, in this particular situation, the MC does not isekai into some OP hero, half beast-man, magical slime, bear, etc. He turns into a brick pooping blob - enslaved by a nasty, grey(?) tribe of brutes. He's forced to live his life eating... what was it? It was either dirt or broken stone. Then, he dies, and his 'soul stone' is left behind, only to be found by the helpers / slaves of an insane scientist. They present this soul stone to their employer / master, and, like all mad geniuses do (obviously), he creates a Frankenstein monster. The body of this monster is comprised of... a human body and crab claw? I can't remember... Moving on, the MC gains consciousness and control over the monster body gifted to him. But, after communicating and expressing his gratitude to the insane scientist, the scientist (being irritated by the lack of stupidity 'his' monster has), gives him a drink that effectively lowers his IQ and functionality. ... That's all I can recall.