Legend unis can also be referred to as "Barnies". The Legends were announced as a new card grade that featured a update to v.5.1.0, they were given out for free with the update, and can be purchased using stars. They can't be posted for sale in the Card Market and can't be sent as gifts.
All-in-all, Legend cards are wastes of space in your lineup and no one should use stars to take their chances on trying to pull one. The chance of pulling a relief of closing pitcher is too high, thus, the chance of blowing your stars is high. Thank Com2Us for another exciting feature to try to entice users to throw money at an average app that really doesn't need these Legend cards at all.