Hi everyone, lately I've been playing the dual necropolis knight light magic army, and it just doesn't work for me - what I'm wondering is: is a chariot king led army(with the death mask) in a unit of 8 - 12 chariots still viable, and if so how would you create a list around it?

You don't need that much chariots. take 7 tops. The point is: 5 chariots with a king and a mask will win combat anyway, and will not be steadfast anyway, which is why you brought the mask. If the enemy makes his check, he will do so against 12 charriots as well. but so much chariots together become unwielding and redirected very easily.

why not field a king with 5 chariots, and 2x 3chariots with him?

The primary problem with a chariot King is that the opponent, if they know what they are doing, will direct attacks at the King's chariot. It is much easier to wound than the King. And if they manage to do 3 wounds on it, the King is on foot. That either means slowing down your big chariot unit to half speed, or running the king on his own. And running the king on his own generally means he won't last terribly long.