Archaon has Mark of Tzeentch by default, since he has all Marks
He's a Level 2 Tzeentch Sorceror by the way...
and the Knights must not have any Mark of Chaos to fulfill the requirement for Archaon's Sword of Chaos upgrade
IIRC the warhounds only has options for poisoned attacks and scaly skin, but none for vanguard. If there was, I might've overlooked it. Will re-read the army book again.
Anyway, this army is just an excuse for me to paint the cavalry regiment of my whole army. Otherwise, I'll be painting 40+ Chaos Warriors, which can be very monotonous.