Kwi-Sin means the afterlife.
Introduced during an event over Christmas and New Year's in 2000-2001, there was a great deal of confusion at first about this. At first, there were minions all over various regions of the kingdoms, centred around the Wilderness. A short while later, a temple appeared with even more minions and even more strange things inside. At one point Ming-Ken, Ohaeng, and Kwi-sin went to the various guildmasters and "settled the aethers" that had come from them. The people had no idea what was about to happen.
After this, when people wandered through the temple in the Wilderness, they were randomly asked questions about the alignments. If you answered them correctly, you became devoted and a follower of that alignment. After the event, you simply need to go to the various temples and answer the questions. The Kwi-Sin temple is in the Arctic Village.
Kwi-Sin is about the afterlife. They are not evil, or bad, or dark, but just specialize in knowing about the afterlife. This brings them to know and embrace what it means.
For rogues, mages, and poets there are no obvious changes for becoming Kwi-Sin, apart from the general shortening of aethers on most spells. For warriors, your whirlwind remains the same as if unaligned, bringing you to 10 vitality.