The Dragon King core set is a cavalry attack missile with massive attack and health boosts for Cavalry. It has since been outdone by the Frostbitten core set however.
Check out the stats for the Dragon King Core set here.
And load it into the Core Builder Tool here.
A quick look over their stats and they appear to be excellent. Particularly the Warlord Helm Core and the Aloe Staff Core. The new pieces all top the tables in their respective fields as well, including the Mystic Knot piece and Red Envelope piece.
We have also added the new Dragon Fire Gem and it is superb: 25% troop attack is ridiculous as far as gems go. I expect it to be pretty rare…