I was inspired by the ones posted here I have in brackets a rough concept of why the players failed beneath the introductions.
The Dark Portal Opens
You managed to defeat Zathrak the evil necromancer body lies motionless on the floor. The battle was won but you lost the war for you failed to cease the ritual in time. With the last dark sigil pounding like some ancient heart the portal opens. For a moment you look into what can only be described as darkness and madness. You can hear noises like rock tapping on stone. You see pinpricks of light in the darkness as the forces from the other side begin their invasion of the realm. You realize that you will need to find a way to defeat them. Your hearts sink if sorrow at you failure but your bodies begin to warm use as you retreat so you can rally the forces of the realm.
[The idea is that this is a failure during an combat encounter with skill checks to stop the ritual to open the portal. The concept is that the players have a set number of rounds to either use skill checks to close the portal, or use combat to defeat the enemy before he completes the ritual. Now the players must win by open war with the enemy. This could slightly extend the campaign or only be a few more sessions.]
New Car Smell
Yours eyes open as you see the your benefactor looking over you. He seems to be studying you. Somehow you are standing erect but cannot move. You benefactor move out of your sight if checking someone else. Just before the paralysis maddens you, your benefactor moves backwards where you all can see him. You realize you are in his workshop. He pours himself some tea, strangely although you feel as if you haven't eaten in a long time, thirst does not come to mind.
"I'm afraid that you didn't stop the Midnight Maiden in time before she cursed you. You bodies are currently implanted with her dark magic. Lucky for you I had a contingency plan if you didn't succeed." Looking you all over the ancient Elf smirks.
"In a strange way this might be more fitting it was the ancient War Machines of the last age the defeated her last time. You must return to the cursed city and finish her for good. If you wish to retrieve your bodies with time I can remove the curse upon them and then return your souls to them. You have only twenty-four hours to get used to your new bodies so you better start practicing."
With a push of a button you hear chains call you suddenly begin to fall forward. You stop yourself and hear your footsteps, the sound of heavy steel striking stone. As you turn you notice in the mirror your image is not your own. You body is not made of metal and wood. Your new glowing eyes flicker as you realize that you are now in the bodies of the Warforged you met when you first came here.
You notice that your benefactor's back is to you. When he speak again it cracks with sorrow and then with a touch of anger. "The bodies you not inhabit were my friends. They served me longer than any of you have been drawing breath. They willingly allowed me to conduct the ritual to place your souls in their bodies. Now they are gone forever, don't let there sacrifice be in vain."
[Similar to the last one as a skill check failure, the players get to do it over, but this time they do it without their racial encounter powers. However they do get the Warforged ones.]
The Light of Hope Extinguished
You kneel beside the body of the boy. His dead eyes are glazed over, the savior is dead what hope do you have now? Suddenly the room is blasted with light. Though the tears stinging your eyes you can see several silhouettes around you. You legs tremble as you can feel their divine auras peer into your very souls. You brains feel like they are aflame as these deities speak to you directly.
"You have failed to protect the savior, how or why you failed is irrelevant your failure is all that matters." The pain flares as if your mind is at the center of an inferno as you feel that anger in your heads. Still you must complete the mission. But the task shall be far more difficult.
"The dark visitor now has no one to threaten his existence, save you directly. You shall locate the host of the visitor and slay it. If the host is king or peasant, old man or child, friend or foe, good or evil it matters not. For now you shall do this or you shall be responsible for the end of not only your world, but countless others." And just as suddenly as the light appeared it has vanished a coldness fills your soul as you see the device that will lead you to your pray, and the dagger that will say it in the hands of the lifeless corpse of the savior whom you failed.
[The idea is that the campaign ending has changed due to failure of protecting the target. The mission is now not only going to be harder but also potentially morally difficult. Kill one innocent to save the billions upon billions of others.]