Yeet. It's Caelus, that baby alicorn boy.

Celestia, Luna and Caelus grew up on Tempest Mountain just like all alicorns before it was destroyed. Caelus was an unusual bat-like alicorn with equally unusual abilities. Unlike other alicorns, which typically specialize in controlling the conventional forces of nature (ie. celestial bodies, water, fire, lightning, etc etc) Caelus could control the space between objects by producing wormholes, or portals. He also had access to other dimensions through these wormholes, and the act that earned him his cutie mark was the creation of the mirror which transports ponies to other dimensions.

Personality-wise, when he was a child, Caelus was boisterous and self confident. He and Luna shared a special bond, and later became a betrothed couple meant to rule together beside Celestia. Unlike Celestia, who swore to celibacy for the sake of her people, Caelus and Luna were designated to be the future parents of the tribe. While of course this was entirely consensual between them, as it really was part of the culture of the tribe to maintain the dwindling population, there was this pressure for them to stay together. At first this was fine with the both of them. They loved each other deeply, as young lovers do, and were prepared to devote themselves to each other. They were destined to lose their immortality together when the time was right to birth the next generation, but of course, that time never came.

As a teenager, Caelus became more and more distant and visibly depressed. He complained of nightmares and shadowy figures manifesting around him, tormenting him nightly. He became a shut-in, and lost every aspect that Luna loved about him. While she had tried her best to understand his plight, nothing she said brought him comfort. Nobody could help him.

Except for Erebus.