Demon Tattletale.
Aeternus chose to ignore them after they ran away. He didn't care if they survived his flames or not. They had placed all the inhabitants in a neat formation for him to kill all at once. That's good enough for him. He looted the fortress and went on his way.
There was no resistance in the next fortress. The high-rank demons had disappeared or maybe there were none in the first place. He waltzed in causing destruction and wrecking havoc. He was like an angel of death and destruction, the representation of gloom and darkness.
The next fortress he reached belonged to the disgusting worm fly. It was the one that created the explosion that destabilise his pillar of flaming darkness. It wasn't a selfless act. The worm had a clone back at its fortress or a life-saving method. Aeternus was sure of it when the soul of the worm tried to escape even before the explosion occurred.