so my guess is he actually died when he fell into the valley, and that eye thing (which was in his shadow even before his hand got marked) was probably either some ancient necromancer's soul or possibly an evil god, and whatever it was, it revived him and gave him the absorb corpse skill, maybe it is possessing him and gave him power for the sake of its own escape or perhaps just on a whim to see what happens (not as likely if necro soul, but more likely if it does turn out to be an evil god) and the black knight sentry was put there by whatever otherworlder sealed said being in the valley in the first place. it probably let MC go since he is in control of himself, but gave him the resist warning cause he could go out of control if he embraces the power too much
...or I could be reading into it too much and he just got an OP but risky power to offset his crap level cap?
my view would be go wild absorbing stuff but also train my skills, it's no problem even if you go mad, as long as you maintain self awareness and a decent degree of self control