The reason why it's hell: Siberia. Not frozen, there are no ice caps anyways, but volcanic...super-volcanic. It was erupting pretty much the whole time Pangaea existed. It, combined with potentially a impact somewhere else, another super-volcano also erupting(both were of the giant pool of lava and magma covers large area...and whats left of the Siberian eruption after 250 million years of erosion covers about a 4th-3rd of Russia. think about that), global warming of a already warmer world on steroids, the oceans dying because of lack of oxygen, bacteria becoming dominant in the ocean that spewed extremely potent greenhouse gases( in two ways: very effective at being greenhouse gases and well...the planet smells like a rotten egg), the planet suffered the largest mass extinction ever. It took so long for life to recover that it got hit by another mass extinction pretty the same time they did.
They dont call it the Great Dying for nothing.