Casimiro was born to Lanathia and Sanctio Est Liberi Dies. He was born with a twin sister, Mariposa, who was actually his half-sister as both of them grew up and started to represent their respective fathers. Casi always considered Mari's dad, Underoath, as his uncle, and she did the same for his dad. Casi's mother was a part of the Versai at the time they were born. She nursed them in Rosemonde where she lived until they were weaned and then they left to go live with their fathers in Desolate Earth. Their mother still visited them frequently enough when she could - she was very busy with her work with the Versai. Eventually, Casi and Mari grew old enough to begin training with their fathers and training they did, for month and months.

It was concerning when their mother stopped showing up to visit them, but they figured that she was off on a top secret mission for the Versai. Casi continued his training alongside his sister and eventually started venturing out to the Free lands to meet others, including two fillies named Raaksha and Keille, and a mare named Rylie. He found he really enjoyed talking to others and finding out their stories.

He was two years old when his mother came to see them again and confirmed what they already knew - that she was on a Versai mission and it had gone badly so she had to go into hiding for a while. But everything was fine now. She wanted them to meet her new friend, Gaddath and their new sibling, a little sister named Liylah. Casi is ecstatic to see his mother again and to meet both Gaddath and his new sister. They all catch up and talk for a while, but things unfortunately go south when Gaddath lets slip that they had been on the run from the Versai. Casi is confused at this, but Oath immediately becomes hostile and attacks Gaddath. Casi yells at them to stop and eventually his mom intervenes - with fatal consequences. One of Oath's hooves strikes her full-force in the chest, breaking several ribs, one of which punctures her lung. Casi breaks down in tears as Elsa yells at them to stop and Sanctio stays with Lanathia as she dies.

Only a few hours after Lanathia is buried, Casi runs away from Mount Vigilum and away from his family. The stallion he considered his uncle just killed his mother. The Versai are just as awful as the Helkaer. His grandmother is Respira, the Helkaer Commander, and his mother was forcibly converted to the Versai and half-blinded by the Versai Commander, Eos. Casi feels his entire world coming down. His plans to become a part of the Versai unravel at the seams and he has no idea how to return home or even talk to his family. He spends three weeks wandering the Free lands in a stupor, barely talking to anyone. Unwittingly, he finds himself in the land of Unclaimed Hearts and meets a stallion named Gidge. The black brute immediately flirts with Casi and Casi returns the affection as best his inexperienced two year old self can. Afterward, Casi decides to seek an apprenticeship with the Vigilum. He meets with a Vigilum mare named Zana's Rose, who takes him as her apprentice. However, only weeks into his apprenticeship a full spot for the Vigilum opens up and he is chosen for the spot. He is honored by the opportunity and determined to do whatever he can to help make Wild Equines a more peaceful place.

He first meets with Zana, who introduces him to the rest of the Vigilum members. He also wanders upon Alastor, Zana's son. He ends up following Alastor to the place the earthquake happened in the Free lands and realizes that a crack in the earth revealed precious gems. Alastor braves the crevice to retrieve stones for himself and for Casi - a sapphire. He then meets with his sister, Mari once more, after she finds out that he's become a Vigilum. He gets a bird companion then - a female goldfinch. A month after he's become a Vigilum, he meets with the Light King, Coreion to teach him herb knowledge. The Light King suggests naming his goldfinch Sol and Casi agrees. He meets with a stallion named Cade, who comes to Mount Vigilum. He then hears the news of Chaos's departure from Neutral Queen from his goldfinch, and decides to go and see Alistair, the Neutral King, on his first venture out as a a Vigilum. Mount Vigilum sees a visitor in Evangeline, one of the Versai members. He then goes to Lachlan Breck to visit Xerxes and Akroma. He is gathering herbs when Sol tells him of a land opening in the Light Kingdom. As he contemplates claiming it, he meets Alastor on Mount Vigilum. Afterward, he decides to claim the Light mainland in the Light Kingdom to serve as a sanctuary and a shelter for the Vigilum and former Vigilum members and their families. He names the lands Omashu and takes Zana's children, Vesuvius and Alastor, his mother's friend Elsa, and his half-sister, Liylah, to the lands with him. On his way back from the Light Kingdom to Mount Vigilum, he is interrupted by a strange feeling tugging at him and is drawn to an area of the Free lands where a terrible mudslide has occurred. He is joined on his quest by Zana, as well as members of the Helkaer and the Versai. The three groups are each drawn to their own rivers. Casi and Zana follow the river to a pond, where it compells them to drink. Zana does so first and Casi finds himself chanting in a strange language. When it's his turn, he drink and it's revealed that the powers of the Vigilum come not through the Afterlife, but through these rivers of power that are connected to the Afterlife. He and Zana leave then and try to process this new knowledge.

Vesuvius and Alastor make an appearance in Omashu to join Casi. Ves becomes Beta mare of Omashu and agrees to help Casi with the border patrols. Gidge then comes to visit Casi in Omashu. The two of them talk for a while and Gidge asks Casi to be with him. Casi says yes. Casi is called to a meeting by Zana to meet the newest Vigilum apprentice, Solara. After that, he meets several new faces in Omashu, including Indiana, the leader of Circle of Stone, which is a subherd underneath Omashu. She brings her two children, Candy and Indus, with her. He meets Sioux Lyon and Phobia, two herd members leftover from the previous herd. He greets an intruder named Snowflake. He then meets with Nixi, the Light Queen. A few weeks later, another approaches the Vigilum for an apprenticeship - a stallion named Rocco Katrini. He is then visited by Coreion once more. Casi and Zana are then visited by Sly, a member of the Helkaer. During a horrible snowstorm in Omashu, Casi is greeted by a large draft stallion. Elsa also joins in the meeting and is horrifed by the draft stallion. She then informs Casi that the draft stallion is named Carnallust the Charmer, a notorious slaver who owns a Dark land named The Whore House. Casi is doing a border patrol when he's greeted by a mare named Evensong. Casi then ventures to the Neutral lands and goes to Shadow Canyon, where he meets Abysm. What starts out as an innocent visit to notify herd leaders of the Vigilum and their abilities turns into a rather abrupt escape when Abysm reacts violently to knowing Zana's Rose is nearby. Casi leaves Shadow Canyon and goes to Mount Vigilum to notify Zana of what's happened. Zana explains what happened and goes to check up on Link in Toxin Raj and tell him what's happened while Casi does the same with Ves and Alastor. Things escalate quickly from there when Zana notifies him shortly afterward that her grandson, Void, has been kidnapped and she's to meet with Abysm in Mount Malvolia if she ever wants to see him again. Zana organizes a group of them to confront her brother and his likely Afterlife help. Delilah, Link, Wolf, and their wolf pups all go to find Void while Casi, Zana, Respira, Ves, and Alastor to go Mount Malvolia to confront Abysm.

Casi is nervous the entire time and watches Abysm and Zana confront one another. He and Respira are then attacked by Abysm's accomplice - a demon named Catalyst, the one who assaulted Zana and is the father of Ves and Alastor. Casi and Respira manage to fight the demon, even as he exerts his demonic illness powers over them. Angry that he's losing to the pair of them, Catalyst leaves the two of them and runs off to the fight between Zana and Abysm. Casi desperately runs after him and discovers in horror that the three of them have fallen off a ledge and close to the lava. He and Respira race over to the edge and see that Abysm and Catalyst have landed on a ledge and are now fighting, while Zana is struggling not to fall into the lava. Respira tells Casi to help Abysm while she helps Zana. Numb, Casi does as she says and slides down to the ledge to help Abysm fend off the demon. With the two of them facing Catalyst, the demon finally begins to lose. Casi lands a blow that breaks one of the demon's legs and together, he and Abysm shove Catalyst into the pool of lava. Unable to watch Catalyst's demise, Casi turns away and instead begins to examine his injuries as well as the injuries from the rest of the group. Remarkably they are all alright. They then need to go and check on how Void's rescue mission has gone. The mission was also a success. Casi is relieved that they managed to secure a victory and hopes that Zana and Abysm can find a way to reconcile.

Casi then gets a visitor to Omashu named Lady Noire, who is an apprentice of the Versai. He also meets Perihelion, the lead of the Light land Jade Mire. He is told of a gathering in Kunipar Cliff and goes there and meets various equines. They discover a mysterious plant there but don't find out about its origins. Casi is doing a border patrol when he runs into Valeria, one of the newer members of his herd. He and Zana then decide to go to Coreion about the Versai and Underoath. The Light King is shocked to discover what Underoath and the Versai have done and promises that something will be done about both him and the Versai. Casi is then stolen by Gidge to Trembling Shadows as a romantic gesture. Gidge has set up a romantic picnic in a valley strewn with flowers and apples. They enjoy several hours of a date night before Zana comes to interrupt it, thinking Casi is in danger. This was unfortunately, not how Casi wanted to tell Zana about his mate. However, all misunderstandings are corrected and Casi and Gidge are left to their romantic night. Casi returns to Omashu in the morning, promising to see Gidge more. The Neutral Queen, Laqueta is then stolen by a Dark land leader named Eleazar. Casi uses his Vigilum privileges to negate the steal and goes to Ittudale in Laqueta's place. Eleazar refuses to hold Casi because of his respect for the Vigilum, so he lets Casi go right after their discussion. A few members then come and go within the Vigilum, including two mares named Solara and Sidava. However, the third member to join their group is the charm. A stallion named Duramus becomes the third member. They also take on an apprentice named Cassia, though she is only an apprentice for a few weeks before she departs the position.

The Vigilum are visited by the Lady of Utopia, Rasa, where they have a brief discussion. An announcement is made throughout the Light Kingdom. Casi attends it as a mainland leader. Coreion and Nixi announce that various concerns have come up about the Versai and they declare that the Versai are banished from the Light Kingdom. They are no longer allowed on Light Kingdom territory and anyone from the Light Kingdom is not allowed to confer with them. Sanctio - who has come to represent Desolate Earth in lieu of Underoath - raises protest against this. Casi bites back, confronting his father about Underoath and Lanathia's death. However, before things get any further, the meeting is concluded. Casi is angry and frustrated, though also satisfied with this new development. As it turns to summer, Omashu experiences a flood, with the mountain stream swelling beyond its banks. Fortunately, it only lasts a few weeks and the land goes back to normal. Casi is traveling through Nomalanga one day when he meets a stallion named Cassian. He tells Cassian about the Vigilum and also about Omashu and invites him to come live there. Cassian says that he will. Only a few weeks later, Casi meets Cassian in the Vigilum grounds again, where he discovers he's become the new apprentice of the Vigilum. Casi is overjoyed at this and welcomes him. The four Vigilum then hold a group session to share skills. Casi meets a Neutral landowner named Chime, who has taken over for Abysm and is therefore Zana's niece.

An announcement goes out that Alistair is stepping down from his position as Neutral King and his son, Avinash is taking his place. Casi, along with the rest of the Vigilum, attend the coronation and wish Avinash luck. Gidge also attends the coronation to Casi's delight and the two of them get to talking. During the coronation, Casi speaks to a mare named Danika, who is curious about Elianna, Gidge's elephant companion, as well as Casi and Gidge's relationship. She is new to the lands and Casi explains how the lands operate, what the Vigilum are, and about his relationship with Gidge. Shortly after the coronation, Gidge decides to move in to Omashu and leaves Trembling Shadows to Azriel. Casi is overjoyed to have his mate living with him now, though he worries that Gidge won't be able to be happy living in the Light Kingdom. Casi then decides he needs to start going to therapy sessions now that the Vigilum have a member that specializes in mental health in Duramus. He asks the stallion about it and Duramus agrees. The first session is difficult, but it goes well as Casi begins to heal from the trauma of his mother's death. Unfortunately, his mental state is further tested as Underoath is brought forth in a trial over his lands by the Light Royals. Casi is brought forth as a witness and mainland leader and his fellow Vigilum, Duramus is brought forth to validate the witnesses are telling the truth. Nixi and Solovey present Kraken as another witness to Underoath's crimes, as his father The Surgeon was murdered by him, as well as several accusations of rape. The Light Royals ask each mainland leader to vote on forgiveness, punishment, or exile. Casi manages to testify, though doing so is traumatic as he recounts the day of his mother's death in front of the Light Kingdom. As a mainland leader, he votes to have Underoath exiled. The rest of the mainland leaders agree with Underoath's exile. Underoath himself is then brought forth and he makes further confessions of crimes, which only further Casi's horror and anguish. Underoath agrees to leave the Light Kingdom while Casi's father, Sanctio, takes over his lands.

Casi is relieved that Underoath has been exiled, but he's in a fragile mental state afterward, having increased anxiety and bouts of anger. Gidge tries to help him through it and Casi is grateful for his mate's support. When the Dark Royals, Sephora and Khalon disappear and new Royals replace them, Gidge occupies Casi to the coronation. Queen Thomasin and King Joker become the new Royals for the Dark alliance. Casi manages to socialize enough to meet with the Dark King to see what he's like. The two have a pleasant conversation before Casi leaves the coronation. Casi meets a mare named Melyra on Mount Vigilum, who asks for help with her brother, Warrior, who suffers from nightmares and PTSD. Casi tells her he'd be happy to help and she needs to tell her brother to come for healing and therapy sessions. She promises to do so and leaves. Casi then meets a stallion in Omashu named Espoir, who is Elsa's friend. He then meets Danika in Omashu, the mare that he met from Avinash's coronation. He is then made aware of the fire in Toxin Raj and Respira's horrific injuries. This causes the Helkaer Commander to depart from the Brotherhood and also to bring Delilah, Link, Void, and Wolf to Mount Vigilum. Casi encounters Void looking for bones on the mountain. When Void tells Casi that he's been having nightmares, Casi tells him that he should see Duramus for a therapy session and agrees to teach Void about herbs. Zana then tells Casi that she plans to leave the Vigilum to explore other opportunities. Casi suspects it has to do with Respira being free from the Helkaer and wishes the two of them well. Cassian then visits Omashu finally deciding to take Casi up on his offer to join the herd. Zana then makes her official departure announcement to him and Duramus. The two of them bid her farewell. They then ask Cassian to be a full member of the Vigilum and he accepts. The three of them travel to the River of Justice to bestow Cassian's Vigilum brand. Casi also meets with a Versai mare named Luna, who explains that she had met previously with Zana to learn about herbs. She also asks Casi to talk about the future of the Versai. Casi decides to travel to the Caerali Desert to see if he can make it through to the Outlands that the Helkaer call home. He is met by Undead Legion, the General of the Helkaer. Surprisingly, Legion entertains many of Casi's questions, giving the Vigilum a better understanding of how the Helkaer work and their mindset. Eventually, though, Legion's patience wears thin and he asks Casi to leave. The stallion does so, grateful for everything he did manage to learn.

Another coronation is announced, this time the coronation of the new Neutral Queen, Orel, who is another of Abysm's daughters, and Zana's niece. Casi attends and congratulates the mare on her new position.