Once Trailblazers have achieved full-star records in Stage 12 of Memory of Chaos, Stage 4 of Pure Fiction, and Apocalyptic Shadow at Difficulty 4 (these can be completed in different versions), the new challenge mode will be unlocked.

"Anomaly Arbitration" consists of three trial stages and one final stage. Each trial stage requires one team (total 3 teams required) whilst you can use any character for the final stage.

Rewards include special leveling items, limited-time avatar frames, redeemable outfits, and more.

Link: https://www.hoyolab.com/article/40294524