It takes away a curse. The old ヴァンパイア want it so I'm guessing it's so they can be human. But they haven't really told us what curse just that it takes away A curse.
the moonstone binds the sun and moon curse. ヴァンパイア can't walk in the sun and 狼男 can only change on a full moon :) if ヴァンパイア break the curse first, which is what klaus is trying to do, the ヴァンパイア get to roam free in the sun but most importantly it keeps the 狼男 from getting their advantage which would be to turn freely :) if 狼男 break it first vice versa
Theory #3: He/She who possesses the moonstone, is in total control. The moonstone could give the holder the ultimate power over 狼男 and vampires. Having similar power to the Gilbert device, it would explain why George and Katherine both wanted it so badly. Reasons why it could work: The moonstone could very well give Katherine the one-up on not just the Lockwoods, but on EVERYBODY!!! And we thought Katherine was tough to beat before! Reasons why it probably won’t: She would be utterly unstoppable. At least to the supernaturals on the show… but wouldn’t it a dramatic twist if the only person that could stop her was none other than Elena? The moonstone could give advantage to Katherine but it would be too much to say it would make her immune to wooden stakes through the ハート, 心 as well.
