Within the death domain, Derik could do and undo, he could create and destroy! To be able to battle him in this domain, one requires to be at a higher level. The earth and fire dragons would have been able to pull this off but unfortunately, the side effects of using their second forms have started to kick in and at their current state they could barely maintain this form. The fact that they were even able to gather enough energy to attack stunned Derik and this increased his urge to draw these beasts over to his side. Unfortunately for them his method has always been crude and sly!
What keeps a kingdom together is not love and respect, these emotions can only take you so far, such emotions can easily be quenched at any moment. Only fear is absolute! The power of fear can never be underestimated, Derik intends to create a reputation where even after his death, he will still be feared greatly and people will be too scared to mention his name.