"Another Story" is a new form of permanent content, similar to Selection Quests". However, Another Story is available in the "Farplanes" in the usual "Events" section and does not have any unit limitations unlike Selection Quests.
Warrior of the Crystals
This is the first one that is released, and only the first chapter is out on August 17th, 2021 (JP). A new chapter is planned to be released monthly for a total of 5 chapters.
Clearing the first mission gives the unit Lucio as a reward, as well as his shards from the mission rewards. Clearing the entire first chapter gives a total of 600 shards.
Lucio is the main protagonist of this story, a Light bruiser/tank that will get a unique passive by progressing with the main story, allowing him to change one of his attack move from Slash to a Pierce or a Strike type attack.
The last mission for the first chapter gives the last 150 shards and is a solo fight, so you'll have to raise Lucio to clear it.