A complete sample of 12 very x-ray luminous galaxy clusters at z >0.5

We present the statistically complete and cosmologically most relevant subset of the 12 most distant galaxy clusters detected at z > 0.5 by the Massive Cluster Survey (MACS). Ten of these systems are new discoveries; only two (MACS J0018.5+1626, aka Cl 0016+1609, and MACS J0454.1-0300, aka MS 0451.6-0305) were previously known. We provide fundamental cluster properties derived from our optical and X-ray follow-up observations as well as the selection function in tabulated form to facilitate cosmological studies using this sample.