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 his Three Corpse and became one with the dao that he cultivate.
To become one, the seeked position has too either be vacant or vacanted (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 come 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 do not come without dangers, as one fused with the Dao that he 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 only has his head that remains 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).
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.