Ephraim Black was the alpha of the werewolf pack the last time the Cullens showed up, along with Levi Uley and Quil Ataera. He and Carlisle negotiated a treaty, which is still in force despite the fact that it is considered a legend.

Look at how threatening he is

How it became a legend within three generations is unknown, but it may have something to do with short term memory loss or something equally convenient.