Sier did not seek the guardian's help or the legacy because he knew how it deteriorated his soul and left him vulnerable. His ailment was a parasite seeking to poison his mind with thoughts whenever Sier lets his guard down.
However, perhaps ironically, the process was slowed down by the insight ability given to him by his bloodline and the guardian. Nevertheless, Sier was able to see the true essence of his legacy, and this was enough for him to seek a method to reclaim his body. Who would be a better candidate for that goal other than the enemy of the Seer Guardian, the outsider himself?
His alliance with Arthur made things more difficult for him, and the fear he felt from the Seer Guardian was the greatest progress he had had in years. This made the ancient god all more focused on claiming his body.
