Skyscraper
Adventurer
Thanks for replies, funny and serious 
The game session was played tonight. I opted for something very close to what @Tony Vargas suggested in post #2. The lord of the tower, I made into a Death Knight. He introduced himself, then said: "the conversation we are going to have right now will decide whether you live or die". They met him in his war room, where a map of the region with forces as they were 500 years ago, as he told the story of the siege on his tower having occured in a specific point in time in the history of the world, and in short the bugbears back then had been called upon to aid the lord and had refused (the bugbears are a weird race in my world, I've renamed them trollocs (like in the wheel of time), and they are savages that hold on to a very strict discipline and as such they are not truly evil, more like lawful neutral creatures, now however duped into following the demon - anyone still following this? lol Anyway it makes sense to my players
). The PCs finally made a deal with the Death Knight: two of them sealed a pact of blood, by which they promised to avenge the Death Knight by killing the trollocs, in exchange for their lives. They even got powers and magical corruptions in the deal, and a magic sword of vengeance for the fighter. But now they have a geas, they must fulfil their part of the deal and kill the trollocs. And, the interesting part is: the trollocs are not truly evil. So they'll end up with a moral dilemna at some point.
And on the way out, they were attacked by the savages who were looking for them, and one of them was mounted on a bulette, and the creature killed the PC Wizard.
Cool stuff. Well not that he died. Just a cool game session all around. Nice atmosphere, good laughs, high intensity.

The game session was played tonight. I opted for something very close to what @Tony Vargas suggested in post #2. The lord of the tower, I made into a Death Knight. He introduced himself, then said: "the conversation we are going to have right now will decide whether you live or die". They met him in his war room, where a map of the region with forces as they were 500 years ago, as he told the story of the siege on his tower having occured in a specific point in time in the history of the world, and in short the bugbears back then had been called upon to aid the lord and had refused (the bugbears are a weird race in my world, I've renamed them trollocs (like in the wheel of time), and they are savages that hold on to a very strict discipline and as such they are not truly evil, more like lawful neutral creatures, now however duped into following the demon - anyone still following this? lol Anyway it makes sense to my players

And on the way out, they were attacked by the savages who were looking for them, and one of them was mounted on a bulette, and the creature killed the PC Wizard.
Cool stuff. Well not that he died. Just a cool game session all around. Nice atmosphere, good laughs, high intensity.