@Mazz and I were talking about this song the other day, it's the music that played during the third job promo animation. I believe it may have also been used in an early KMS beta client? Can't remember for sure. Third job was released in December 2005.
Back then, the third job advancement was very different from today's. The advancement required you to pass both a physical and mental test. The physical test is mostly the same, except each job had a separate portal to enter across Victoria Island.
Warriors entered through the ant tunnel (the map that had the 24h Merchant and the entrance to ZMM).
Magicians entered through the highest part of Ellinia, the last malady map.
Bowmen entered through the Sleepywood dungeon, in the map just past Forest of Golem.
Thieves entered through the hidden maps of the Kerning swamps (probably the most hidden of the 4 portals).
These portals were known as "The Doors of Dimension."
Upon killing the clone of your main job instructor (Grendel, Athena etc.) you would be given a black charm. This charm would be used as proof of your win and would be exchanged for the Necklace of Strength.
After killing the instructor clone, you would make your way back to El Nath to take the mental test. This would require you to have a dark crystal in your possession to activate the giant Holy Stone (found in Holy Ground at the Snowfield) here you would have to answer 5 questions whose answer's you should have come across over the past 70 levels. At points when irrelevant questions were removed/new ones were added, the answers were not always correct. If you failed any question of the mental test, the stone would deactivate and you would have to get a new dark crystal. By completing the test, you received the Necklace of Wisdom.
Together, the two necklaces would prove that you were eligible for the third job advancement.
BLAST TO THE PAST.
If anyone know where to find the animation for 'The Third Flight' please share below! :)