My review to Season 1 and 2 are at:
https://mydramalist.com/profile/7403249/reviews/402084
https://mydramalist.com/profile/7403249/reviews/402278

Could Tanya be the descendent of Asa Sin and therefore the rightful religious leader of Arthdal? If so, then the Wahans could also be the descendents of Aramum Haesulla and Asa Sin. This mystery was unveiled in Season 3.

With the help of Taelha and Tanya, Tagon rose to become the Union Leader. Asa Ron was not going to let Tagon rose to power so easily. Tagon hid two secrets and Asa Ron knew one of them and he was determined to expose it to the tribal leaders to topple Tagon. He set a trap for Tagon. To hid both his secret, he massacred the tribal leaders and set out to be a king. Tagon had wanted to become a benevolent leader loved by his people. But this rebellion changed him and he became a tyrant.

Meantime, Eunseom escaped from the mine. He rescued a Momo tribal leader and her baby. The Momo tribe had a motto of returning favours. The tribe became indebted to him.

Unfortunately Eunseom was captured again, this time by the Ago Tribe who captured humans to be sent to Arthdals to be sold as slaves. But by a twist of fate and help of his fellow prisoner, a sly and treacherous man, he became recognised as the Isainagi re-incarnate. The Ago tribe was a lost and disunited tribe. They had been hoping for a powerful leader and Eunseom fitted the role.

Now, two powerful regions were formed. One led by Eunseom and the other by Tagon. Friction between the powers were inevitable. And this was to be presented in the sequel produced 4 years later in 2023.

Might I also add that Song Joong Ki did an excellent job acting a pair of twins with opposite characters - one was kind and innocent, the other ruthless and cunning. I rated 10 for all seasons.

Now I am going to start on the sequel filmed in 2023, 4 years later but covering Arthdal and Ago 8 years later. But in the 2023 sequel, Song was replaced by Lee Joon Gi who is also and excellent actor.