One thing I love about Christian Sahlén's "The Abbey of Saint Wilk" is the unexpected nature of the adventure's main villain. When reading this one page dungeon, I expected a minor or diminished demon to be lurking near its conclusion, and did not see the Grand Duke of Hell coming at all, and your players probably won't either.
What is so special about this place, that warrants such a high profile foes attention? Well there are two clues given in the adventure. The first is in the description of the grand duke himself, stating that he seeks to increase his knowledge of things, and considering a lowly knight rose from the dead to smite him some 66 years prior to the adventure, this could definitely be something he wished to understand.
The other motivation is simply revenge. He seeks to punish those connected to, and around the abby after his resounding defeat in the past. His own flying steed, a formidable foe in its own right, even feels as if this is all a needless distraction and is frustrated by the whole affair, perhaps even to Eligos's peril.
Nice work Christian!