Titian, The Flaying of Marsyas”.
"The story is from Ovid, and it is one of great and sustained cruelty. Though we catch it at a particular moment, we know that this flesh will be long in the stripping and the unpicking. Here is how this monstrous scene came to be. A contest has taken place between the satyr Marsyas and the god Apollo. Marsyas had discovered a set of reeds abandoned by Minerva. He learns to play them so well that he is foolish enough to challenge the god Apollo to a musical contest. Apollo agrees – but on condition that the victor will be able to inflict such punishment as he chooses upon the loser. Predictably enough, Marsyas loses, and Apollo inflicts his gruesome punishment, which is to flay Marsyas alive, stripping flesh from bone, inch by meticulous inch.
Ovid barely mentions the contest, though he does describes the scene that Titian paints in some detail: “‘Help!’ Marsyas clamoured. ‘Why are you stripping me from myself? Never again, I promise! Playing a pipe is not worth this!’ But in spite of his cries the skin was torn off the whole surface of his body: it was one raw wound. Blood flowed everywhere, his nerves were exposed, unprotected, his veins pulsed with no skin to cover them. It was possible to count his throbbing organs, and the chambers of the lungs, clearly visible within the breast.”
- from independent.co.uk's article on the painting.