No special rule for that Illumini.
I understand you are talking about the death of Asavar Kul descibed in some books... but there is also another -most epic- version of his death. It appears in an ancient White Dwarf (about when GW released Hordes of Chaos army book) and explains the youth of this Chosen One and how he conquered leadership of his tribe (the Kurgan). In this version, it was Magnus the Pious himself who defeated Asavar Kul in a duel. Much more epic... but also much more unreal. Magnus was a priest with high leadership skills, and some kind of fighting skills... but obviously, very far from any chosen of Chaos, not talk in comparison of Asavar Kul itself.