This page of the guide to Elex 2 contains a walkthrough of the mission Willpower and Transformation. You will receive it during the Adversity quest. Your will have to go to Betara and meet Tayaac.
Where is Tayaac?
You must go to Betara, a location that you have already visited during the Antibiotics mission. Go to the orange indicator - that's the entrance.
Go deeper into the location, then enter the tunnel and keep going forward. When you reach the fork, take the path to the right and then take the first left turn.
At the next fork, turn right and follow the tunnel until you reach the room shown in the above screenshot. Go towards the purple flames - this is where you will find your mission objective.
Meeting and fight
Go up to the landing and talk to the character named Tayaac. During the dialogue, the mission will end and you will receive another main quest - Traitor.
After the conversation, Tayaac will attack you. He's quite a strong opponent that deals high damage - make sure you dodge and block him. After the fight, you have to go to the exit - teleportation will be blocked for the time being. You'll soon see a cutscene, and Jax will be moved to the exit.