rami.ananygw said:

i can't find how and where you can build such units please help a beginner

You cannot build legendary units in the same way as other units in your castle. You can only buy them on the Black Market using sapphires, or get them as rewards on the Saga Quest (while advancing on the battlegrounds found using the Eagle's Nest). Presently I'm a level 58, and I have received a total of seven War Elephants and two Pegasus units. And it looks like once I complete Saga Quest level 10 (which is up next for me), I will get 3 The Fallen units and 9 Hydras. However, one has to remember that these legendary units can also die in battle if the opposing forces are large. So far three of my seven War Elephants have died. So be careful when sending legendary units into battle. They need to be accompanied with enough supporting units.

thx alot for ur clarification i dont get it about the support unit can you enlighten me about them??

Lord Bunnyonwheels is correct you can only buy legendaries with sapphires at this time.  Great western Dragon supports Dragons, wyverns, chimeras dark ones also. You can see on the picture when you click them what they support.