Realms, both wondrous and dire, border the world. One such realm is the Shadowfell. Although not inherently evil, the Shadowfell is fraught with dangers, and the barrier between worlds can be thin. Sometimes the darkness breaks into the light. Nearly two hundred years ago, a cult of the demon prince Orcus purposely created such a gap, connecting one of Orcus’s unholy Shadowfell sites to the world. Skeletons, zombies, and fouler creatures flooded through the rift into the light of day. The empire of Nerath dispatched legionnaires to eliminate the threat. The empire’s soldiers destroyed the undead, sealed the opening. Time passes and empires fall, and even Nerath wasn’t immune to the ravages of time. The once-great human empire collapsed almost a century ago, and only memories of its glory remains.

As the adventure begins, the player characters find themselves travelling down a road that hasn’t been maintained in nearly a hundred years ago. Explain to the players that their characters know each other and are travelling through a dangerous wilderness toward the small town of Winterhaven. The four of you have made the journey to Winterhaven for two reasons, firstly your friend Douven Staul the explorer travelled this way to explore a rumoured dragon tomb nearby. Nothing has been heard of him for some time, which does not bode well. Secondly Helmclever was contacted by a priest of the deity Pelor called Marla. She has heard rumour that death cultists have been seen in the area and are practicing their unholy ceremonies in secret. It is speculated that a dangerous and twisted priest called Kalarel may be involved. You have been charged with investigating these rumours. If they are confirmed you are to destroy this cult and all its activities.

After that, take a look at your wiki. There is some more helpful info there.

Keep On the ShadowFell