Bragging Rights.

PRIDE chased him, but it couldn't catch up. This is because the two of them are both moving at the speed of light. PRIDE can become much faster than that, but not on the light side of the void universe. The speed of light is the limit on motion on the light side, so unless PRIDE could weaken Aeternus by making him lose energy and unable to maintain his speed, it wouldn't be able to catch up to him soon.

The alternative is for PRIDE to chase Aeternus until Aeternus runs out of energy. PRIDE went with the alternative. It didn't have a choice. It had tried to infect Aeternus with his energy during their fight, but it was like giving Aeternus free food. It was gobbled up as soon as it reached Aeternus's body.