'Hehe. That show-off! He didn't know how the ritual worked.'
There was someone in the Durbag tribe who thought of this when he saw Eren getting selected for the final ritual. He laughed on the inside while maintaining serious expressions on the outside.
Durbag had participated in this time's Oni festival under the false identity of Borul. He had used special means available to him to pass off as Monster Lord. He had regressed into his evolution stage to pull this off. Otherwise, the ritual would have detected an anomaly in him.
Varhan was asking himself to be part of the final ritual by being so connected with his audience. Since their emotions were focused on him the most out of all the members of the Durbag tribe because of how interactive he was, his selection was a foregone conclusion.