This cultivation level is the last stage to cultivators to achieve their true immortality, in reality a Dao Ancestor is a Zenith Heaven Immortal who managed to slay their Three Corpses and became one with the dao that they cultivate. They have to transcend the Heavenly Dao Tribulation in order to officially become a Dao Ancestor. Their law threads, soul and nascent soul fuse with the body and they attain golden meridians and jade bones.
To become one, the position has to either be vacant or vacated (death of the previous one, abandon the position or become a Primordial Chaos Dao Ancestor), because there is only one Dao Ancestor for each of the laws of heaven and earth meaning that if the position is occupied one has to either kill the actual Dao Ancestor of the law of heaven and earth that you cultivated to take his seat or one has to steal it by force (because you possess a better comprehension of the law than the Dao Ancestor that you want to replace) before the Dao Ancestor of your law of heaven and earth comes to kill you, in any case when one aims for a position already taken it is a situation of life and death.
Unfortunately, being a Dao Ancestor does not come without dangers, as once fused with the Dao that they cultivated, one can be swallowed by it, forcing all Dao Ancestors to not use their true power frequently. The more they use their power of laws, the more they are assimilated by the Heavenly Dao until they became one with it, meaning death (the Dao Ancestor of Divination/Clairvoyance only has his head remaining from using too much of his Dao Ancestor power and even the previous Dao Ancestor of Time, The Ancient and Present One, the most powerful Dao Ancestor to exist still lost his legs from using too much of his power).
Every Essential Dao Ancestor has a corresponding, subordinate Dao Ancestor.
Currently, there are several hundred known Dao patriarchs in the True Immortal Realm, and every single one of them had to endure countless tribulations and hardships while also encountering incredibly miraculous opportunities to reach their cultivation base.