A Challenger Approaches is a side quest given by Nuru when she's returned to the outpost after the Ceremonial Hunting Caverns mission. Players do not have to complete the mission first, if another character has returned Nuru to the ark it's immediately available for anyone when visiting there.
In a new universe the quest Fetch the Floran Artifact will have to be completed before this quest is available.
For this quest, Nuru asks the player to travel to fight in an arena. Speaking with Nuru again will open the option to teleport there. Once inside are three challengers, a melee fighter, a bruiser and a ranged shooter. All three must all be defeated before a passage opens to return the player to Nuru for turn in.