hey guys I cant post in the other sections yet so hoping some one here can help I got this cacti the other day it has black spot on top....do I need to cut this of is it some kinda fungus and if so how much to cut the whole top?

Interesting looking cactus. What exactly is it?

If it's mushy under or around the back area I'd cut out the affected area, or even just cut the whole top back to just under the affected area. If, except for the black part the cactus flesh seems firm then I'd leave it alone. I have several cacti which sustained damage or minor rot that stabilized and worked itself out. If the black area spreads or any part around it gets soft then cut it back.

Damage to the tip like that is often caused by physical injury (like being banged around inside a shipping box) or by sunburn (on a plant that is newly introduced to full sunlight). In the winter it can also be caused by freezing temps. Do you know what caused that damage to the tip?

Wolf gave you good advice. From your photo it looks like the black area is not soft or weeping. But if it is then stop watering altogether and hopefully the black area will harden up. If it remains soft or starts to spread then cut the tip off. But if it is hard and not weeping then just water the plant very lightly until the black area heals up and scabs over. At that time you can resume watering normally. Best luck.

THANKS GUYS!!! so at first it was still hard but it did start to get a lil mushy (SLIGHTLY) especially to one side of the black so it was cut out...not the whole top just half of it there were a couple smaller spots that werent mushy so i left them. It is a t. bridgsii (probilly spelt that wrong) a lil unique as i havent seen others of these get so thick then thin then thick but from reading it seems maybe who ever had it before me could have been keeping it outside in summer and inside in winter suposedly that can cause the fluctuations in thickness....but again new to this so not 100%

Oh 1 more thing will the cactus bruise a lil kinda like mushrooms? the piece was cut out with a razor there was NO black left at all! looking now it has a couple streeks of black where the razor cut into it is that normal or does that mean i left some of the infection behind? i tend to think its just some kinda bruizing cause it is in the spots where the razor cut and in straight lines (like you would make the cut) but again still new

They vary in thickness depending on how much water, nutrients and especially light they are getting at different times. When you see columnar cacti that are evenly thick it means their conditions have remained fairly consistent. Lots of mine have varied in thickness as from season to season I've moved them in and outdoors and found them more or less sunlight and played with fertilizers and soils.

Most people sterilize their cutting instruments with Isopropyl Alcohol before and between cuts. Some dust the cuts with sulphur powder to protect infection. Sharper instruments cause less tissue damage and bruising.

Blowing a fan on it will help the cut callus faster. Sulphur powder or cinnamon applied to the cut right after surgery will provide some protection against infection.

If you're not sure that its healing properly then post a photo of what it looks like now.

Depending on the humidity level of where you live, if its mushy it may be prone to rot. If this is the case it would be better to sacrifice that piece and use some sulfur dust to over the fresh cutting to prevent rotting (standard cactus propagation technique). I have had a similar thing occur to me and it hardened up within a month, although i am in texas and this was in summer.

That's one cool plant, any update? I hope it's doing better ..
Personally I would of cut it out and let it dry like you did.