Please note this next part is for Bishops mainly, but also gives a description of the stages and what you have to do.
To activate the quest....
Talk to Robertia who is the 3rd job Magician instructor. There will be a series of travelling around that you have to do while talking to various people. Some of these people include Wiz the Librarian Finding various people and talking to them, including Wiz the librarian,Shammos, and Grendel the Really Old. Eventually, Shammos will give you the Genesis quest. He will ask for an item called a "Life's Root", which is found at the Horntail Quest. Head over to the Cave of Life and talk to Moira. The quest is called Secret Medicine of Transformation, which requires 1 morrow (cube), 1 broken dagger, 1 skull mask, some tough skins, and 1 Manon's Cry.
Pre quests...
You need to get 6 people who have done the quest too. Exactly 6. When all the members gather, complete the quest. You will then be transformed into a Green Lizardmen. While in this state, you can't attack. If you leave, you will need to do the quest again. Once you are in this state, Drop down and the guard will recognize you and let you inside instead of usually denying you.
Horntail Quest.....
There are 7 stages to this quest, and it can take a very, very, VERY long time
First stage. Kill the Cornians in the map you enter to obtain Keys. You have to have one member of the party wait outside, and this person can't have a key or be the party leader. All the other members who have keys should go to the top left corner portal.
Second, Third, and Fourth stages. One member should complete the jump quest while the others will jump down and kill the monsters to obtain a key, then complete the jump quest also. The key should be dropped at the shiny spot next to the portal, then it will be sent to the first map. The member who was left behind should get the key from the middle tree, then talk to the NPC found below. Repeat this for Stages 2, 3 and 4.
Fifth stage One member will complete the jump quest. The others will go down to get a key, then complete the jump quest. The key will have to be givin to the party leader, then that person has to go through the portal.
Sixth stage You will need to talk to the Horntail Schedual NPC, then all the members of your party will be teleported to a little place called "Cave of Choice". The party leader has to hit the object on the left or right, then go into the portal. You will then either be sent into "The Cave of Light", or "The Cave of Darkness".
Seventh stage Well in whichever cave you were sent to, kill all the monsters and get the 6 keys. Give them to your party leader if that isn't you. The party leader has to then climb over the high wall and talk to the NPC, and the other members will kill the monsters that are there. When the monsters are killed, the party leader has to talk to the NPC. Then you will be teleported to the Horntail map finally. Wait there until other parties have finished the quest, because Horntail is impossible with only 6. Note: At every Stage, the monsters you kill have a chance to drop Life's Root. Look out for them. Once every Bishop in your party gets Life's Root, you can proceed out. The next stage is the Horntail fight (head) itself, which you would not want to do.