It isn't in the tech tree. It used to be founded by thefairly early game technology Awaken the Ancients, which was availible only to the Sheaim or Kuriotates. At that time, the 2 dragon units (Abashi and Eurabates, not including the BArbarian dragon Acheron) could only be built in the CoTD holy city. Now that has been reversed; they can be built anywhere, but the first city to build one of them becomes the holy city of the cult. Since the dragon require the very late game technology Divine Essence, the spread rate had been greatly increased and the dragons where given the evangelist promotion (which spreads the units religion into any city it conquers, but hasn't been implemented yet in the BtS beta)