To start the quest you need to talk to an NPC dragon in Yalahar magician quarter named Tamoril it will say about the other 4 dragon bosses.
Tazhadur is the first one and you only need to kill 200 dragons to get the access to the boss, the boss teleport is in Darashia dragon lair.
Kalyassa lair is in Dragonblaze Peaks and to get access you need to open 5 chests
Gelidrazah the frozen lair can be found in Okolnir and to access you need to answer 3 questions to the dragon statue next to teleport.
Zorvorax lair is in the vampire crypt to get access you need to walk in 3 places: the sun tower in Ab’dendriel, the oasis in Darashia and the stone circle in Carlin
After killing all these bosses and getting the 4 items of each boss: tooth of Tazhadur, horn of Kalyassa, scale of Gelidrazah and bones of Zorvorax, you get access to the first dragon boss.
The First Dragon
The First Dragon lair is on Ankramun mountains and need to use levitate to reach its spot, there you will need a team of at least15 people separated in 3 groups of 5 people to use the lever. Each group will need to defeat a Fallen Challenger after deafeting it will turn into a teleport and send all to the main room.
In the room you have to lure a creature named Spirit of Fertility in a Heaven blossom located at left side of the room after doing another creature will spawn called angry plant kill it to turn into a flower bowl and lure one of the 5 Unbeatable Dragons turning it into a Somewhat beatable repeat all this process until to kill all dragons. After this 6 dragon essences will spawn to kill them quickly focus in killing just one because if you kill one all die, after all this you can finally kill the boss The First Dragon and after killing it you will be teleported to reward room.
You can kill all bosses again after 20 hours!!
Reward after killing the first dragon:
Festive outfit (ask to NPC The First Dragon to get it), a Porcelain Mask (you can get one after 5 days of the first time killing the boss),3 Colourful Feathers (you can get once a day) and a backpack containing : 3 Gold tokens and Ancien Coin, Draken Sulfur, 2 Seacrest hair, 2 Mydtical hourglass, a Blue gem, Red Gem, a Yellow Gem,2 Demon Horns, 2 Slime heart, 2 Energy Veins, 2 petrified screams , 2 Brimstones Shells, 2 Deepling Warts, 2 Wyrm Scales and 2 Hellspawn Tails (you can get the backpack once a year).
For more info: https://tibia.fandom.com/wiki/First_Dragon_Quest