Prerequisites: The Blue Crystal (Demona)

There are two spawns with Lava Dragons. One below An Old Dragonlord in Draconia and another in Fibula. The one in Fibula requires a golden key to enter.

Player: hi
An Old Dragonlord: GEETINGS MORTAL! HAS OIN SENT YOU? OR HAVE YOU COME TO ASK FOR A MISSION?
Player: mission
An Old Dragonlord: Did you kill 400 LAVA DRAGONS?
Player: yes
An Old Dragonlord: EXCELLENT... YOUR BLOOD IS TRULY THAT OF A DRAGON... MAKE YOUR PREPARATIONS AND ASSEMBLE
  YOUR TEAM.. THIS WILL BE YOUR HARDEST MISSION YET! WE MUST MAKE OUR MOVE! CONSUME THIS CRYSTAL AND YOU WILL
  HAVE THE ENERGY TO PASS THROUGH THE BARRIER! GO INTO THE CAVE BELOW AND DESTROY OUR FORMER BROTHEREN THAT
  HAVE FALLEN TO THE DARKNESS a dragon blood.

Consume the crystal by using it.

You feel a radiant energy rush through your body.

This will give you access to Zathroth.