This feather given to you by Sora after defeating him when playing the re:||VERSE story campaign (any route) on Hardcore mode (limited COINZ). This item can only be obtained and used after seeing at least one of NOISZ re:||VERSE's endings. The feather allows you to select the normally grayed-out option, “I'm going to save everyone.” The main character calls upon Sora to help them, and Sora obliges.
This puts you into a series of battles, known together as Stage X8. Once entering X8, the stage number is replaced with two question marks, and COINZ are no longer limited. However, an accurate count of COINZ technically remaining in the run will still appear on the save file. This can go into the negatives. All fights but the last are ones you did not complete previously in the lead-up to reaching X8.
Clearing X8 unlocks D.S. al fine on the difficulty played and all those below, a special ending, and (for the first clear) the achievement “One Last Thing.”
Trivia
- In one of the beta builds of NOISZ re:||VERSE, it was possible to obtain Sora's feather before clearing the re:||VERSE story campaign at least once. Players who obtained the feather in this beta were able to use it in the final build only after meeting the clear requirement.<ref>https://twitter.com/noiszgame/status/1207790475266183173</ref>