If you've not completed Hordes Of The Underdark yet but have ambitions to do so, go away now. Otherwise I'm seriously going to spoil it for you.
Last chance now.
I managed to complete Hordes Of The Underdark last night, with about an hour's gaming time to spare. Shockingly, it doesn't end in a hugely anti-climatic battle, like most RPGs. Instead, you get to duke it out with an Archdevil in one of Waterdeep's courtyards, who does all sorts of nasty things like trying to turn your henchmen against you, and summoning lava and ice elementals to try and drain your hitpoints (making environmental resistance items pretty much compulsory). Once you have sufficiently beaten Mephistopheles to a pulp, he'll surrender and beg you not to kill him. "Please, your honor, spare my life" to which my Evil Ranger replied "There is no honor" and promptly killed him. Interestingly, you don't actually need to fight him at all, if you get his True Name a little earlier in the game from The Knower Of Names (at the cost of 500,000gp!), and you can simply banish him from Toril. I (or rather Thaeonia), however, wanted blood. After all, she proclaimed "This world is mine!" before she slaughtered him - as it's not exactly a wise thing to do, leaving an Archdevil at your back nursing a grudge...
Once the battle is over, it segues nicely into an epilogue that concludes the story and tells you what happened afterwards to all the characters involved. It's definitely going to be worth replaying with a neutral character and a good character, to see how these accounts change. Convincing Aribeth to stay evil once you rescue her spirit from Cania has pretty nasty consequences for Neverwinter (she goes off afterwards to haunt it and cause all sorts of mayhem), but it was nice to see happy endings for Sharwyn and Deekin. Obviously treating people differently or not doing certain actions, such as betraying The Seer or using a different second henchman instead of Sharwyn in the first chapter will alter the tales. It should be fun to experiment and find out just how mutable the ending is. I'd say it's a far more satisfactory ending to both of the previous single player campaigns.
Though it's possibly not as great as The Sims 2, which should be waiting for me when I get home tonight. I'm going to get this pun in before everyone else does for my prediction of what it's going to be like: SIMSATIONAL!