In addition to being episode 3 of King's Quest from waaay back (see King's Quest 3: To Heir is Human), To Heir is Human is the last of the three modules made by the folks at Bioware for that delicious soul sucking game they call Neverwinter Nights. The other two are The Winds of Eremor and The Dark Ranger's Treasure. In a lot of ways "To Heir is Human" is the best of the three.
It's suitably difficult, perhaps not as much so as the quite-frankly-insane Eremor, but definitely enough to give a level 5-15 character a run for their money, and definitely enough that I'd recommend taking along a friend for this one, since unlike The Dark Ranger's Treasure no henchman is provided. Its worth noting that unlike Eremor, they at least supply a shop in this one.
The plot still dosen't get a chance to shine (you're not really playing a little module like this for plot are you?), but it sets up the quest in some very cool ways. Its a surprisingly complex bit involving a deal with the devil and drow armies. When its over you'll wish you could continue your interference in it a bit longer so you can find out exactly what was motivating all the players in this little play (it definitely leaves a bit of a cliffhanger, and could invite a sequel).
It's also the longest of the three by far, probably larger then both the other modules put together, and there is even a bit of a subquest in the dungeon. Everybody whose played through the official campaigns of Neverwinter Nights and is looking to get a little more play out of Bioware's lovely toy has no excuse for not giving this one a whirl.
Of course, it's still not half as cool as Witch's Wake, but thats a whole other story....