Here is the guide:
To start the Quest you have to find the PreRune:
After you destroyed the PreRune, 2 Red Runes will appear on the map.
Both of the runes need to be destroyed. After that you need to find the IceFrogs which spawn on the map. They are invisible and you`ll need a gem to find them. Also they are unkillable by Physical attack and only magical attacks are available. So you can use Dagon for the purpose.
Here are the spots where you can find the IceFrogs:
After you killed all the IceFrogs, suddenly you will be killed and a message will appear: "Player has been killed by Phoenix". Well done, if that message appears, the Phoenix will appear at the boss spot on the right bottom of the map. Prepare before the battle, because the boss is very powerful:
The Phoenix got 20,000 HP and very fast regeneration. He has a special skill, summons a Super Nova. When the Super Nova is falling, all units will be stunned globally for 10 seconds. After you killed him, he will transform into an egg. But you didn`t win YET. The Phoenix reborns even more powerful!! Here you`ll probably need a cheat to kill him! EDIT: in the comments Timo says that using Guinsoo before killing the first Phoenix... will prevent him from spawning again.
After killing the Reborn Phoenix you`ll get 5 of those nice presents:
Ancient Medallion gives you +75 all stats, 9 illusions (active), 3x critical and some evasions.