The Legend dragons are dragons who have the Legend element. There are Legendary, Wind, Crystal, Mirror, Droconos, Nirobi, and Robin Hood Elements. They can all be bred except Robin Hood.
To breed a Legend dragon, you need to use parent dragons with the Pure element. So Pure dragons as well as any Pure Hybrids will work. Legend being a wildcard element also counts as Pure.
When both parents have the Pure element, then a Legend can be bred. The odds will always be low, and so you should expect to have to try many times to get a Legend. Since the Light and War update, I have tried over 60 times, with only 3 Legends (Crystal, Nirobi, Mirror) to show for it.