Curse of Kalarel
Shadowfell keep is located near Winterhaven.
Whilst investigating the troubles there the party came across this decrepit structure and deemed it worthy of further investigation; they could sense dark energies emanating from the place.
Here they met Splug, the plucky goblin that broke out of the racial stereotype to win all our hearts, fighting evil where it comes.
Here they also met (and defeated) Kalarel, the evil priest who was trying to open a portal to the Shadowfell.
Or so they thought.
With his last breath (whilst being pulled into the rapidly closing dark portal he had begun to open) he cursed the party and marked their flesh with his evil tattoo.