adds a new island to the world of Vvardenfell, but this is no vacation spot. The local populace there is vehemently anti-social, though you can get quests from them and ultimately end up going down one fork of the storyline, which is the main quest, with them hand in hand. Their behavior is justified, though, when you find out that the other half of the locals suffer from a bad case of turning into werewolves.
This brings about the other branch of the storyline, where you, as a creature of the night, can put aside all of your earthly belongings night after night to wreak havoc on the townspeople. And you won't get off trying to be a nice werewolf, either. You periodically have to dine on the flesh of man in order to stay alive, but the trade off is uncanny physical strength and really cool, nasty sharp, pointy teeth.
Bloodmoon's story is well written to tie in various rivalries and conflicts on the new island of Solstheim. The Empire has established a new mining colony there, which causes a little bit of friction with the Viking-like locals. Both prongs of the expansion, the main quest involving the colonists and the werewolf path, are deep and add quite a bit more gameplay to the original title.