Party kara Tsuihou sareta Sono Chiyushi, Jitsu wa Saikyou ni Tsuki S01E06 - Regarding the Quest, Which Is, in Fact, the Start of a Comeback

Laust and his companions enter the labyrinth on their quest to battle the phoenix and earn the bounty from the Guild. They quickly track the phoenix down and engage it, but the instant they start causing damage to the fiery creature, it begins to mutate.