The Father of All Linnorms is already pretty powerful, but is he enough of a challenge for a group of mythic heroes (20th-level with 10 tiers)? How would you go about "buffing" this already impressive beast?
I suggest putting on some mythic ranks else he will go Down to the first mythic Spell or ability the heroes use on him.
I would most likely make a flesh start and costum make him as a mythic monster.
What sort of mythic abilities do you think such an ancient linnorm would possess?
at least he need some of the mythic save stuff. he will have like 4% chance of surviving a mass suffocation cast by a mythic level 20 wizard that means it.
if you look at the mythic dragons in the book and look over the mythic monster options i am sure you will find some to suit your taste.