Luther

Luther, with Cypher Zahariel in the background

Luther was the best bro of the Primarch Lion El'Jonson before the shit hit the fan in the Horus Heresy.

Back Story

Luther was a Knight of The Order of Caliban, an organisation dedicated to protecting the people of that world from the beasts that roamed its forests.

One day when riding through the glen with his merry band of men, he encountered Tarzan a naked wildman who had no business living alone in that environment. So they captured him and brought him back to civilisation, educated him and introduced him to their order, where they called him "Lion, Son of the Forest".

Together the two warriors rose up the ranks of the Order and became living legends, eventually coming up with a plan to rid the whole world of the beasts and make it safe for everyone. This took some time, and eventually the Lion got promoted to the top job of Grand Master and Luther became his assistant. This is where the cracks had started to show, as many had thought that Luther was on track to gain the position, if the Lion had not been discovered, and that Luther was one of the greatest men of his time, only surrounded by even greater my by circumstance.

Shortly afterwards the Emperor arrived and brought the Imperium to Caliban and told the Lion of his heritage and his destiny to lead the First Legion on the Great Crusade.

The Lion took many of his Knights with him on this quest and although Luther was too old to become a Space Marine he was still augmented as much was practical and was brought along as the second in command.

This didn't last long though. Luther inwardly admitted his envy of the Lion and his Astartes brethren, and during a mission to Sarosh contemplated allowing an enemy nuclear bomb to detonate aboard the Dark Angel's flagship so he could take his rightful place in history.

However he did not follow through with this thought and successfully rescued the flagship and exposed the Saroshi plot.

Despite the Lion apparently never knowing about Luther's private doubts, he sent him and select others back to Caliban in order to fortify it and train new Dark Angels for the ongoing crusade.

Exile on Caliban

Back on Caliban, Luther became heavily dependent upon the Librarian Zahariel and the Cypher.

At this point, something had begun to twist inside Luther his behaviour became erratic.

Learning from Cypher's collection of dusty tomes about rituals and incantations, he uncovered a "plot" where Terran colonists were apparently attempting to summon a creature called the Ouroboros onto Caliban. With Zahariel's assistance he broke up this coven and attempted to gain power over the creature for himself.

Zahariel discovered that this plot was actually false, that the Terrans were in fact attempting to banish the Ouroboros instead of summoning it, and that Luther was looking to gain power in an attempt to free Caliban from the oppression of the Imperium.

Later, Luther captured one of Malcador the Sigillite's Knights-Errant: Garviel Loken, who were on Caliban to discover where the exiled Dark Angels loyalties lay, since no contact had been made from them in some time.

This would prove to be a turning point, as Luther and his allies had no idea that the Horus Heresy was taking place around them and had been kept ignorant by the Watchers in the Dark and one of their Astartes allies (it is not explicitly stated to be Cypher, it makes more sense to be Zahariel considering the circumstances and his own connection to the Watchers)

The Knights-Errant killed a Dark Angel during their escape, which would upset the balance of ignorance that had been maintained and set the Dark Angels on the path for the Fall.

The Fall of Caliban

When the Lion return to Caliban after the Horus Heresy had finished, he found his world that despised him and the Imperium and a war took place which cracked the very core of the planet, leaving nothing but The Rock.

According to the legend, Luther had ascended to daemonhood and was now more than an equal for the Lion, and eventually defeated him, though in a final moment he realised his error and broke down in tears.

When the Dark Angels descended upon the site of the battle, the could not find the body of their Primarch, the only clue was given by Luther who had said that the Watchers in the Dark had taken the Lion away.

41st Millenium

Luther is still alive 10,000 years later, kept in a cell at the heart of The Rock known only to the Supreme Grand Master of the Dark Angels. The cell is only unlocked using the Sword of Secrets and it is the duty of the holder of that blade to force Luther to admit to his crimes and own up to where he hid the body of their Primarch.

Suffice to say that no Supreme Grand Master has succeeded, though during his interrogations, Luther is convinced that the time is nigh when the Lion will return and forgive him for his betrayal.