Immortal Body was the main perk that any humans and elves alike would receive the moment they managed to break through to the illustrious Rank 6 magician. However, the prerequisite that individuals from each race needed in order to be established as a Rank 6 powerhouse was different compared to the other.
As for the Elves, they had the prerequisite of successfully breaking into the stage 5 of Katra.
Breaking through the said stage meant individuals had successfully converted their spirit core, where it had become the home to their soul. It also meant that their body had turned into a mere vessel for their soul, an ageless artifact that was continuously tempered by their spirit core.
With the completion of this achievement, the said individual could be considered as immortal among their peers - a figure that stood above others.
Another info for my reader who follows Earth's greatest Magus. In this world, as they were cut off from the outside world for a long time. the word Magus was considered ancient.