The Key is the ninth quest in the Main Quest Line, continues after the quest Into The Jungle.
Upon turning in the Poachers Lure to the Fisherman, he will provide the coordinates of the island with the stump on it and the Strange Compass. The player needs to go on that island and use the Strange Compass to find the said stump. Loot it to find the Strange Key, which can be used to open the Strange Box.
You will find the Tome of Trials, wich describes the items used to summon Draegan.