Great Chaos Dragon

Scaly Skin gives 3+ saving throw

Daemons are not of mortal flesh and have some immunity to mundane weapons. Daemons get a 5+ Ward save. However, this Ward save cannot be used against magical attacks (le. spells and magic items).

Unit causes Terror.
Troops who are confronted by monsters or situations that cause terror must test to see whether they overcome their terror. If they fail, they are completely terrified and are reduced to gibbering wrecks. Troops only ever test for terror once in a battle. Once they've overcome their terror they are not affected again.

If a creature causes terror then it automatically causes fear as well, and all the rules described for fear apply. However, you never have to take a Terror and a Fear test from the same enemy or situation - just take a Terror test: if you pass the Terror test then you automatically pass the Fear test, too. As any unit of Troops only ever takes one Terror test in a battle, any subsequent encounters with terrifying monsters or situations will simply count as fear.

A unit must make a Terror test if charged by or wishing to charge an enemy that causes terror.

A unit must make a Terror test at the start of its turn if there is an enemy which causes terror within 6".

The unit do not take Panic tests.

Unit can fly
https://6th.whfb.app/flyers

Add +1 to be hit for shooting

An Emperor Dragon counts as having a unit size of 30

The Chaos Dragon’s has two different breath attacks. It may use both in the same turn.
Each turn, one head breathes fire, with a Strength of 4 and the other breathes a cloud of corrosive gas, resolved at Strength 2 with a -3 to Armour saves.

960 points. The Great Chaos Dragon counts as three Rare choices for a Hordes of Chaos army.