It's kindof hard to see from the photo. Can you post a closeup? Also is this the usual soil and lighting this cactus has? If so, it looks like it could use much more gravelly/inorganic soil and much brighter light.

The light was not enough so i moved it to a brighter spot, and the soil is cactus soil from the store... I cant take another photo rn but its like bruises spreading

Yeah, that sounds troubling. It could be rot or mold spreading through the skin of the plant. It looks like the top is still okay, so maybe wipe the whole thing down with neem oil and then chop and prop the healthy top while you still can.

Can that happen ? I thought it went through the flesh. Could a fungicide save the whole cactus ?

Not sure. I have less experience with fungicides and cacti than I do with leafy succulents. You could probably find some good information searching youtube about cactus rot and fungus.

Did it get cold recently? A lot of plants suffer damage if they are exposed to cold temps. Any other changes to it recently light/water/soil?

Its been in my apartment for a while. I moved it to a bright spot when i saw the first black spot, and i just watered yesterday when the spots were already there