Nitric acid is considered to be a very strong oxidant and a very strong acid. If human flesh is exposed to nitric acid the effects would be catastrophic. Nitric acid will cause the skin to melt exposing the bones below the flesh and sometimes even dissolving the bones.
Recently 25 prisoners were executed by throwing them into the pool of nitric acid and leaving them there till the time their bones and their bodies dissolved completely. These prisoners blamed for spying.
Nitric acid is generally used in the production of the fertilizers and leaves serious burns and blisters when comes in contact with the human skin.
This execution was conducted by a terrorist group and was conducted in a public square so that everyone else is warned.
This undoubtedly is the most gruesome method of execution that anyone can ever think of. Imagine having no remains left for the family to pay the last respects to.