Created in the infamous 21st Founding, the Voidborn are a loyalist chapter with significant deviations from the Codex Astartes, stemming from their unusual mutation and the superstitions derived from it. Their progenitor primarch is unknown; speculation sometimes names the White Scars, Dark Angels, or Iron Hands as their founding chapter, but it is more likely that they have chimeric geneseed, and for their part the Voidborn show no interest whatsoever in the identity of their primarch or the rites of his legion.

The Voidborn's oddities in organization and chapter cult stem from a mutation in their geneseed affecting the Lyman's Ear and, it is believed, the Sus-an Membrane. The normal functions of the Lyman's Ear are unaffected, but it also, when a Voidborn is in interplanetary space or in the Warp, emulates the third eye of the Navis Nobilite. Through some accident of genetics, the Voidborn can hear the light of the Astronomicon as a perpetual choir.

This has several effects on their order of battle and chapter culture. They prefer to fight ship-to-ship battles over planetary warfare, and as a result are extremely skilled in zone mortalis warfare and the cleansing of space hulks; accordingly, full battle-brothers will all train in the use of terminator armor early in their tenure, though when deployed in large numbers terminator armor will be reserved for veterans, as in other chapters. Because the Choir of Terra cannot be heard when inside a planet's gravity well, it has also given rise to a superstition among the Voidborn that it is the height of bad luck to set foot on the ground of a planet under any circumstances. Accordingly, their order of battle when in planetary combat relies heavily on bikes, land speeders, and transports. Through service done for the Adeptus Mechanicus and securing complementary STC data which filled in details the Mechanicum had not previously known, they secured their assistance in devising a custom transport built on the Rhino chassis, known as the Choir-Pattern Rhino, which is longer and has more roof hatches and side firing ports, so that it can carry more marines and allow them to conduct the battle without disembarking; these are their preferred transport, with half or more of the Battle Companies deploying within Choir-Patterns in most engagements.

Due to this superstition's incompatibility with close combat, being deployed dirtside as an assault squad is considered a punishment duty. This being so, assault squads are usually fielded with a chaplain in place of or in addition to the squad sergeant, and the Codex Astartes 8th Company (Reserve, Assault Squads) is absent from their order of battle. It is not replaced with a company of some other squad; their geneseed has instability beyond the Choir mutation, so they are perpetually under-strength. In years of relative peace, they maintain about 600 battle-brothers, organized as one Veteran Company (Silver), three Battle Companies (Red, Green, and Black), one Reserve Company of Tactical Squads, fully trained in deployment on Bikes, Attack Bikes, and Land Speeders (Orange), one Reserve Company of Devastators often deployed on Attack Bikes (Blue), and Neophytes in the Scout Company.

Scout Marines, who are not full battle-brothers and for the most part cannot yet hear the Choir of Terra, are deployed on foot as in codex-compliant chapters. Notably, unlike most chapters Voidborn scouts rarely if ever are fielded on bikes, highly-mobile marines already being abundant. Neophytes are sometimes also fielded armed for close combat, Fenris-fashion, though this is done only when the commanders have reason to believe there is urgent need for close combat forces, since the casualties from close combat in carapace armor are too high to sustain for ordinary operations.

Beyond their superstitions, the Voidborn's culture reflects their shared experience of hearing the Astronomicon as the Choir of Terra. All Voidborn learn to sing in harmony with their brothers and the Choir, and most also learn improvisational harmony and write usually-wordless hymns, sung in battle, during rituals, and whenever the mood strikes them. They have an unusually large fleet, especially for a perpetually under-strength chapter, and take a more active role in steering their ships, allowing them to maintain fewer Navigators per ship. The Navis Nobilite leadership are known to be suspicious of them, fearing those who could replace them, but for their part the Voidborn display a quasi-filial reverence for Navigators, as someone who can see the Holy Light they grasp only dimly by ear. A few minor Houses supply most of their fleets, and the long-standing relationship means that many of the navigators have passed down Voidborn hymns as family traditions.

Their chapter cult treats the Emperor as divine, though not in a sense the Ministorum recognizes. It treats the Astronomicon as an aspect of the Emperor, much like the Tech-Priests treat the Omnissiah as an aspect of Him, and emphasize the Emperor's guiding vision for mankind and His guiding song for all those who travel. Theirs is an intimately personal worship, since while away from gravity wells they hear the Emperor's song always; they are most likely exceeded only by the Custodes in how closely they feel personally tied to the Emperor Himself.

Colors: Black with golden trim, except the left leg which is gold with black trim.

Symbol: A silver field bearing two barred sixteenth notes in black, with a golden lightning bolt crossing them, splitting the notes' bar in two.