From what I remember the MC is male and the world changes with monsters appearing but they can't leave the mountains for some reason(maybe a barrier) and people start either devolping special powers or eating special plants to get powers the MC lived near a big mountain or mountain range but I can't remember how he gets powers I think he travels around trying to get treasures like fruit or plants to get stronger and for some reason I remember a monster cow or ox that appears early The MC at the start finds a flower while hiking that is taken by a bunch of op monsters but touches some pollen/petals and that how it starts at this time monsters had not started to appear I think I know this is all very vague not holding much hope
Sorry for late response and no I don't remember any fine details I just remember the start and some weird details like how he is hiking and finds a magical flower that a bunch of animals are fighting over and touches a drifting petal and I think the pollen on it is special or something I know it sounds weird
Had a look but I don't think that is it the novel is set on modern times I think and the MC is a normal person who at the start is hiking and comes across a magical flower that animals are fighting over he touches a petal that drifts off the flower while the animals are fighting and starts to devolpe powers I think?
That's not it but still the immortal ruins looks really interesting thanks I remember even though the world changes the monsters/evolved animals were trapped in the mountains because of something like a barrier I think those who tried to exit suddenly burst into flames and died except for one with a strong bloodline/potential
Epoch of Twilight is the closest to what you described that I can think off. It does not meet all your requirements though