Likely a shortened form of Sebastian or Cassian, from Latin roots associated with reverence or belonging to Cassius.
Castian is most naturally understood as a variant of Cassian, a Latin name derived from the Roman gens Cassia — an ancient patrician family whose name may connect to the Latin cassus, meaning "hollow" or "empty," though other scholars suggest an Etruscan or pre-Latin origin. The Cassii were a prominent Republican-era family; Gaius Cassius Longinus, one of Julius Caesar's assassins, is perhaps the most historically notorious member. In Dante's Inferno, Cassius is consigned to the lowest circle of Hell alongside Brutus and Judas — a measure of how profoundly the betrayal of Caesar was mythologized in medieval and Renaissance culture.
The name took on spiritual resonance through Saint Cassian of Imola, a 4th-century Christian martyr said to have been stabbed to death by his own students with their metal styluses — a grim but vivid legend that made him the patron saint of stenographers and teachers. John Cassian, a 5th-century monk whose writings on monastic practice bridged Eastern and Western Christianity, gave the name lasting theological gravity. Castian, with its softened vowel shift from Cassian, feels more contemporary without abandoning the name's classical architecture.
The -ian ending aligns it with a constellation of rising names — Dorian, Florian, Lucian, Julian — that blend Roman gravitas with modern accessibility. In fantasy literature and gaming culture, Castian has also gained traction as a character name, lending it a slightly epic, world-building quality that appeals to parents drawn to names that sound both ancient and adventurous.