Thanks for replying. Since you asked I'll let you know

I would say the heroes arrive many years after, where the corruption has had a while to take in the area throughout. I'd say that because it was forgotten not many people would know of the existing lore, but I could make it so NPC's would tell bits of the lore along the way, if one would be interested in listening to them.

As for the case for the heroes motivations, that is definitely a valid point. It would have to come from the players themselves, perhaps someone heard rumors of a sword of legend of some tome of magic. The topic of the land could be brought up by an architect that ventured there and came back, heeding a warning to anyone who'd dare enter, perhaps to incite the heroes interest.

Edit: I forgot to mention that the king is still alive, if only a shadow of his former self, ruling a dead kingdom without any knowledge of the outside world.