Unfortunately, it was quite hard for Nial to find a drawback in Dilan's Origin ability.
It looked like a drawback didn't exist, especially since it had already been several days since their fight had begun and Nial was yet to find one.
Under normal circumstances, both Nial and the Undying should be dead tired by now. However, Nial used the mana he had absorbed from the surrounding area to keep himself fresh.
His darkness domain continue to expand and he used it in a very precise manner so that Dilan wouldn't take note of anything. That may not be necessary because Dialn was too focused on his battle with Nial, to begin with, but it was better to be safe than sorry.
'He is still as strong as he had been in the beginning…' Nial figured quickly, unable to comprehend where the Origin of his powers came from. It was impossible to regenerate out of nowhere. The Undying had to have a source of energy that allowed him to heal at such a monstrous speed.