Is the 'bite' area soft or squishy? I'd suspect rot. The potting soil looks to be the standard peat based potting soil used for most houseplants but really a very poor choice for any cacti or succulents. Much too moisture retentive. They would prefer one with a minimal amount of peat or other organic matter and mostly mineral based ingredients to provide the very fast drainage these plants require.

Oh awesome! i had a feeling the soil wasnt doing great with them becasue soem other smaller ones around that one hadnt really rooted well. I’ll work on some new soil. Can i remove the top healthy part and work with that to salvage the cactus or is the poor guy a goner?

You are wonderful thank you so much

You can chop it off with a clean knife at about the halfway point. I wouldn't chop it much lower as it's getting thin there. Maybe at some point it didn't get enough light. Just make sure it looks fresh and green inside. Let it harden off a day or more until it's calloused over. Use commercial potting mix combined 50/50 with perlite and a small pot. Don't water for a few weeks, keep in bright light but not direct sun. At some point you might be able to move it into direct sun. My 2 cents.

Aside from the issue with the bite taken out of the plant, there seems to be a big problem with the amount of light the plant is getting. This looks to be a Mammillaria which usually has a globular shape, only elongating with age (there are exceptions). Yours looks far too columnar in shape for such a plant. I suspect you are not giving it enough light. The growth pattern already shows a lack of light where it is thinner but this does not appear to have been corrected enough. Just compare the size of the tubercules at the base of the plant to the ones at the top.

That area looks like it was cut out with a knife. Are you sure that someone did not already attempt surgery to remove a rotting section on this plant?

I am concerned about the tiny sliver of brown that travels vertically that I have circled here. This looks like a rotting section. If that grows then you will have to cut off the roots entirely and keep cutting up the stem until there is no rot in the cross-section. You will have to sterilize the knife after each pass as well.

The soil is potting soil, too moisture retentive for this genus. Try to find some pure pumice, sized about 1/8th of an inch. Do NOT include the pumice fines. Try to let the plant recover in that pumice medium, and one advantage of pumice will be that it is easy for you to uncover the affected area and check on it periodically. The pumice will not stick to the base. Be sure to bare root the plant and wash off all of the existing soil before planting in pumice.

Was the plant inside or outside?

tj