Krokodil is most accurately used as a term to describe the incredibly crude desomorphine product produced by red phosphorous/iodine reaction (Nagai reduction) on codeine. The hegative destructive effects to human tissue are no doubt a mixture of symptoms resulting from toxic phosphorous/iodine species present (some phosphorus compounds cause bone toxicity, hydrogen iodide present is a nasty acid, presumably there might be phosphine or phosphorous triiodide too... nasty)
Desomorphine is the primary active alkaloid in the mixture. When isolated on its own, as a pure alkaloid, it's Just Another Opioid with nothing unusual about its toxicology.