No, you can't. you need defeat all 3 warzones bosses first to have the chance to fight against Ancient Spawn of Morgathla. also you need 3 items to fight him.
Mallet Head, dropped by The Baron from Below from Warzone 4;
Mallet Handle, dropped by The Count of the Core during Warzone 5;
Mallet Pommel, dropped by The Duke of the Depths during Warzone 6.