Someone who transcends becomes more like nature than their former selves. When something becomes made up of more nature energy than life energy they become a fey - a process called Transcending.
When a druid has held the title Nature Protectorate for 200 or more years, they are granted the power to transcend by a group of higher powers called The Balance.
A druid does not have to be a Nature Protectorate to transcend. An alternative way is to take a druid prestige class or be exposed to some powerful artifact or spirit of the land. Black Wood is thought to have transcended from exposure to the fell energies of the Tarfilerquar.
One of the bad effects for transcended humanoids is a reduction in their birthrate. This is normally a fifth of what is normal for their race.