From what I gathered, the medal in the alpha for not dying (perfection) was the Mark of the Unbroken medal. My thoughts are they either changed the name of the medal at one point and forgot to update it everywhere or as another commenter mentioned, "Legend" may have been the class of medal so basically saying, "get a uber rare medal" and getting 15 kills and 0 deaths could have been the only Legend class medal in the alpha. Who knows. It kind of makes sense when you look at the other medals. The medals seem to follow a similar color coded system like the gear. The easy to get ones are white (common), the harder ones are green, the special ones are blue, and then theres the bright lime colored Mark of the Unbroken one. If you find a screenshot or video of someone getting a legend medal though, post it here. I've been looking everywhere for one.

This is a screenshot of the bounty.

Cool thanks! I think Legend may refer to the class of medal, (common medal, uncommon medal, rare medal, legend medal) like common, uncommon, rare, and legendary weapons. Either that or they changed the name of it somewhere down the line. Here's a video of a guy completing this bounty (24 - 0) and he gets the Mark of the Unbroken medal. http://youtu.be/835c-XhVM18?t=6m30s Another vid of someone going 20 - 0 and getting it too. http://youtu.be/wxJu7MyChw0?t=11m55s