While fighting diablo, if you are trapped in his cage and its that time to where he stuns you and sends you to fight shadow diablo you will not be able to move when you spawn in the shadow realm. I guess that the game reads you as still being trapped in the cage and will not allow you to move or use any skills. You will die very quickly as you are locked in place. Hopefully this can fixed soon as not to cause and hardcore players any problems. Luckily I was playing softcore at the time.