Help me think of a cool way to bring payoff to the "Throne" card

So I decided to put the Deck of Many Things into my campaign where the players are level 6, and it ended up going very well. One person gained a loyal knight follower, one person became an idiot, one person had all their magic items disintegrated (all 2 of them), and one person drew the Throne: "you gain rightful ownership of a small keep somewhere in the world. However, the keep is currently in the hands of monsters, which you must clear out before you can claim the keep as yours."

I told him that he has no idea of the keep's name or whereabouts, just that somewhere out there in the vast world of the Forgotten Realms is a keep that will be his once he defeats the monsters.

What would be a cool way of having him earn his keep in an organic and non-contrived manner sometime later in the campaign? He's already on the lookout for books or scrolls detailing lost kingdoms and dig sites of ruined keeps.

Edit: I meant more on how he could organically discover the keep without it being a "woah this keep you cleared out for part of the narrative happened to be the one!" type of coincidence. I have plenty of ideas for what horrors await within the keep.