Still, no one refuted him. He is their leader and their objective was to ambush Aeternus, then kill him. They may not be able to ambush him, but killing him is still possible. They just had to catch up to him. So they ran after him.
They should be faster than him because of their stats and rank, but reality proved differently. Aeternus, the low-rank demon had a speed equal to mid-rank demons. One of the first lessons that Soverick learned in the academy is about habits and footwork. He learned to walk and run right. The techniques are not transferable because of the difference in anatomy and physiology but the knowledge is, so the techniques can be simulated within his mind and imitated with his body. That is why he has a speed equal to them.
