From Halopedia, the Halo wiki

Fracture will be an special event type in Halo Infinite. These events will reward the player with specific thematic rewards. According to 343 Industries, the rewards from Fracture events will, at times, "lean into things further afield".[1]

During the Season 01: Heroes of Reach, the Tenrai event will be one of such events, with the Yoroi armor being the event's reward.