Lore

So from my understanding

• The Ban Clan used to live in Erinn, then Neamhain cursed them for some unknown reason.
  • In order to avoid their curse, they made Pincara and fled there.
    • Pincara ran on sacrifices apparently. But no one wanted to sacrifice themselves.
      • So Bith sacrificed herself to make a clone making facility and Pincara ran on artificial sacrifices.
        • Of course this wasn't a permanent resolution and an end would happen.
        • Some clones were used to explore out of Pincara and find answers.
        • The clones are apparently half bloods.
  • At one point they managed to visit Erinn again, but only to discover their people was forgotten.
  • The Ban Clan's curse has been lifted with the disappearance of the gods.
• Dian is apparently a god.
  • Cessair's Heart's other name is Cecth. Cecth also happens to be Dian's emotions.
  • She apparently thought that her brother, TH, was dead. One of her motivation for revenge, was due to his survival.
  • She was angered by the fact that her people was forgotten.
  • Dian is grieving with the fact that she killed many to reach the position she's in.
    • And her plans are to make sure that her people's death was not in vain.
• TH is of Ban Clan origin. His mom, Bith. His dad, Birren. His sister, Dian. Yeah you heard me.
  • Birren sacrificed himself to save his three children.

I also assume that the Ban Clan were the first people of Erinn. Cessair in the original story was the leader of the first settlers of Ireland. Dian also mentions that she's older than Merlin, and his 100 year old age is minuscule compare to hers.

Wait? Birren had three children? Well, he saved three, but I don't recall all of them being his. Also, wasn't Dian the leader of the settlers? Not some slave's (Birren) kid?

I realized that the symptoms those three children experienced was a curse on the settlers, a curse from god, a curse of death. - Complete the Passage

But this man had been able to cure his children. The slates specifically state that he saved his children, no one else.

TH, while reading the slates, mentions that someone else wrote some parts, these are most likely by Dian. Meaning someone else was there way before her and TH talks to Dian mentioning "our mother/father".

I'll concede on this point.
Any hints as to who that last kid is? TH, Dian, and someone else...

Nothing about the last kid. It's possible both TH and Dian don't know if the last kid is even alive. Considering Dian didn't even knew TH was alive.