Well in relation to some other topics that float around here I am posting this. Since Skeith is a virus, I kind of made up my own assumption. Maybe it is answered in later games, I don't know. I was just thinking that when you go to fight Skeith there's those waves and all then the final frame is a sort of info screen saying
Phase One: Skeith
The Terror of Death
Maybe the virus evolves from your average monster, then in phases like Skeith, next boss, next, and finally next!
[spoiler]^^^yep, Skieth is just the first phase...I believe Maha is a phase as well but I'm not sure???? The phases were designed by Harold to protect Aura from Cubia....or at least thats what I believe....[/spoiler]