Anger And Calmness.

What they didn't know is that the dragon king is also unhappy with the current situation. He can only watch them. It is grating to him that he can't do anything useful against them.

It is not that he can't. He is not entirely powerless, as there are a lot of things he can do. It is more like he should not do anything to them at all. 

His single eye is glowing with anger and resentment as he stared at them, but he didn't do anything. His mind ran through the possible things he could do against them. He outlined them one by one as he looked at them.

For starters, he could use his cosmic source to overcome the barrier between the light and dark side of the void universe to attack them. This attack is exactly what he did to Legion-8. But this time, it would be much stronger since the barrier of the realm tree is absent, so he would surely kill them.