Just wanted to add to what Aneg said as her answer is wonderful but I did want to touch on a few things.
The Final Dragon and everything that spawns with her during the fight doesn't seem to drop ANY loot. I've participated a couple of years now and it still hasn't changed.
Just wanted to add what you get in the backpack (which is once a year):
Ancient Coin, Draken Sulphur, 2 Seacrest Hair, 2 Mystical Hourglass, 3 Gold Tokens, a Blue Gem, a Red Gem, a Yellow Gem, 2 Demon Horns, 2 Slime Hearts, 2 Energy Veins, 2 Petrified Screams, 2 Brimstone Shells, 2 Deepling Warts, 2 Wyrm Scales and 2 Hellspawn Tails.
You will get the 3 Colorful feathers every day you participate. Also, every 5 days you've participated you will get a Porcelain Mask. The reward of The First Dragon fight seems to be nothing other than what's in the chests, the outfit of course, or if you want to sell the loot from the chest your more than welcome to make a quick profit DURING the event (because after the price will drop at least what I noticed in Pacera)