Hi everyone!
Here my problem: When i push with a medium amount of force into the pedals there is something into the drivetrain that make an sudden huge sound (same sound as the chain when it jump) and let the crank arm spin almost freely for some very little degree.

• When the bike is on the table , the chain seems to run normally whitout any jumps or other problems; (also the chain is kinda new).
• There are no bended tooth on the sprocket;
• Derailleur is working fine , with the correct align;
• Cable and cable holders are new;

I have no idea where to cheeck or what can be but since is the smallest cog that really piss me off (since i really use a lot this one).
Ps. This bike is an mtb with 9 speed deore.

thanks for help.

It seems that one or more of the pawls within the freehub body are sticking.
To repair you will need to be able to remove the cassette and freehub body. This can be relubed or replaced. Unless you have access to the necessary tools then it's a job for your local bike shop. You cannot fix this problem without dismantling.
There's plenty of videos on YouTube to keep you right if you want to give it a go.

I'd suspect the chain is worn or the sprockets first. Also possible that your DR adjustment is slightly off and your chain is not fully engaged with the cogs. Or, maybe a imperceptibly bent hanger bracket for the rear DR.

The freehub is possible too, but I seldom have issues with them.

Does this do it in every gear or just certain ones?
If it does it in all the gears it is most likely stuck pawls.
If it does it in only a couple of gear ask yourself, How old is your chain? How old is the rest of your drivetrain? Were they replaced together?

Only one cog (last one). Chain and the rest of the drivetrain are pretty old but are still not "worn" by use. I think i can clearly do more ~5k kilometers

It's probably worn out.
The smallest cogs have the fewest teeth to spread the load.

In case i need to buy a new one , can i use a 10v cassette instead of my 9v?
ps: here the image of my cassette, is not rusty is just dirt.. when cleaned is shiny as silver

Not unless you also change your derailleur, and shifters. 10 speed MTB rear derailleurs have different cable pull ratio than up to 9 speed; thus 10 speed shifters will not work properly with 9 or less speed RDs.

Have you measured the chain for wear? All it takes is a ruler; along the top of the chain measure out 12 inches (30.48cm) "As a general rule, if the rivet is less than 1/16″ past the mark, your chain is ok. If it’s between 1/16″ and 1/8″ past the mark you’ll likely need a new chain, but your sprockets should be ok. If it’s more than 1/8″ past the mark, you’ll probably have to replace both the chain and cassette." https://cyclingtips.com/2016/09/how-...-chain-wear-2/

There is another possibility: you don't have enough chain wrapped around the 11T cog. This can be due to the chain being too long, and/or the 'B' screw is adjusted too far out.

I had the exact thing happen to me. All components were in good working condition, then one day, I started to get a "skip" in the smallest rear cog. It just didn't make sense - working perfectly then all of a sudden skipping in this one gear. Felt like a worn chain or sprocket - but to happen while riding?!?
Turned out that a small pebble had wedged itself in between the 1st and 2nd cog and wasn't letting the chain fully seat into that gear.
Never had that happen before and obviously an easy fix.
Maybe something similar to this is your issue?
Check for debris between the cogs.

IF your gears are good. You can also check your derailleur alignment to make sure it is not off in that gear. A bent derailleur hanger will cause that problem as well.

Solved , changed the last cog and now everything was fine.... but sadly i spent more to repair this one than getting one new.

That's my guess too. "Smallest cog, use it a lot fairly new chain".

I used to be able to get replacement single cogs but I doubt that'll happen anymore. If one cog is worn that badly, can others be far behind? If it was my bike I'd replace the whole cassette.