T'Challa's Tribal Council
Once he became the new King of Wakanda, T'Challa held a Tribal Council meeting during which Okoye announced that they had discovered Ulysses Klaue had resurfaced and planned to sell a Vibranium artifact in South Korea. The Tribal Council discussed the opportunity to go and arrest Klaue, and despite the Merchant Tribe Elder's disapproval, the mission was decided.
However, the mission failed and T'Challa returned to Wakanda empty-handed. Later, a man arrived in Wakanda and claimed to have killed Klaue himself. The individual was taken to the Citadel by W'Kabi to meet the Tribal Council. It was then revealed that the newcomer was N'Jadaka, also known as Erik Killmonger and son of late Prince N'Jobu, who had come to claim the throne of Wakanda. N'Jadaka challenged T'Challa in the throne room, and T'Challa accepted the challenge.
Erik Killmonger's Takeover
Nakia and Okoye secretly met in the Citadel after Erik Killmonger seemingly killed T'Challa in ritual combat. Nakia urged Okoye to come with her, Shuri and Ramonda, but Okoye refused, stating that she was sworn to serve the new King as a member of the Dora Milaje.
The next day, Killmonger held his first Tribal Council meeting in the throne room and revealed his plan of sending Vibranium-made weapons all around the world in order to trigger a global revolution which would be spearheaded by Wakanda, making them the rulers of the entire world. Although Okoye disapproved this plan, W'Kabi supported it and the rest of the Tribal Council was forced to follow.
Actually, T'Challa had survived his duel against Erik Killmonger. He and his allies managed to defeat Killmonger and his supporters, enabling T'Challa and his family to reclaim their place in the Citadel.