Rise of the Brave Tangled Dragon: Tale of the Black-Eyed Plague

Hiccup, Merida, Rapunzel, and Elsa's kingdoms(/island-dom) are besought with a mysterious and terrifying plague. Their people have become violent, and cannibalistic, doing and saying anything to feed on each other. The infected look entirely normal, Except for their eyes; Black as the deepest sea. 

Hiccup finds a ship from Corona (Rapunzel's Kingdom) full of the infected. After attempting to make peace with the men, they attack, and he is forced to flee. He goes to consult Rapunzel about this, only to find her entire kingdom destroyed and empty. Her Castle under siege of the Infected, Hiccup must find a way to save the innocent people of Corona, while also warning his people, Merida, and Elsa.

Meanwhile, Jack is transported through time and space to a land full of fear and plague. To find the man who sent him there, he must work with Hiccup, Merida, Rapunzel, Anna, and Elsa, to cure the plague and find a way back home. But this will become infinitely harder, seeing as Jack has been turned into a Human.