It's heavily implied that Myste's manifestation was in response to the Warrior of Light's guilt over their inability to save Haurchefant (and to a lesser extent Ysayle). His appearance highly resembles a younger version of him (albeit with Ysayle's hairstyle), he first manifests just outside Fortemps Manor, and some of his lines further suggest this - not to mention one of the Quest Log entries, stained by Fray once again, mentions Haurchefant's broken shield. Count Edmont showing up and briefly mentioning Haurchefant at the end of the questline (when you've returned to the Last Vigil explicitly to try and discern why Myste manifested then and there) further suggests this.