The audio starts with Draigo and Mortarion squaring off, and Mortarion criticising Draigo for identifying himself through names and titles which to the Daemon Primarch are less than worthless. He asks Draigo repeatedly "Who are you?" though what he is really asking is who Draigo thinks he is.

Audio then cuts to the battle with Draigo, who is the 6th Grand Master, fighting with his Paladins against the Daemon hordes. They do well and witness Geronitan move to fight Mortarion, as the entire Grey Knight strategy was based around, but Geronitan has removed his faceplate to meet Mortarion eye-to-eye in an honourable duel. Mortarion's response is to whip up a hurricane of plagued air that infects Geronitan and renders him unable to fight, a Librarian named Torvus saves Geronitan before Mortarion can cut off his head by teleporting him away but Mortarion's Manreaper still cuts Geronitan's throat, and infects him with Nurgle Rot.

Geronitan dies before anyone can do anything to help him, and Draigo calls a Conclave of the Grand Masters. Vorth Mordrak, Drystan Cromm, Vardan Kai, and the other Grand Masters all commune psychically across the galaxy, although five of them are actually at Kornovin, and they decide that a new Supreme Grand Master must be elected. Cromm, the 2nd Grand Master, reveals that the Grey Knight allowed Mortarion to ravage entire sectors in order to draw him to Kornovin where a prophecy claims that a champion will be able to kill Mortarion permanently. But that champion was Geronitan, who apparantely was groomed for that position since he was an aspirant, even his name was a carefully prepared psychic weapon to use specifically against Mortarion.

Without Geronitan the Knights are unsure what to do and decide that the new SGM must decide, stay and fight or retreat and fight another day. Both Kai and Mordrak nominate Cromm who declines, saying he must be the kingmaker. The 1st GM nominates himself and one of the GMs takes himself out of the running as he isn't physically present. Draigo realises that Cromm will never accept, Kai is too young and that the 1st GM and Mordrak will never allow the other one to ascend. He nominates himself and gives a good speech about why he is doing it, and the GM that remains on Titan seconds his nomination and then Cromm thirds it. Everyone agrees on Draigo but in order to become SGM he must swear the oath over the Titansword, which Geronitan dropped before Mortarion slit his throat.

Draigo, Cromm and Torvus return to Kornovin's surface and are able to find the Titansword just as Mortarion arrives. Cromm and Torvus leave to allow Draigo to fight him on his own but Cromm gives Draigo one final weapon. Mortarion's true name, the name the Emperor gave him at birth. The two square off and Draigo mentions Janus in his speech, to which Mortarion claims that he knew Janus and that he isn't surprised that the Grey Knights don't know who the original Grand Masters were before they became "Malcador's errant angels." He says that the truth about Janus would destroy the Grey Knights but chooses not to elaborate and fights Draigo.

Draigo puts up a good fight and kills several Deathshroud prior to actually fighting Mortarion but the Daemon Primarch overwhelms him. But in a moment of arrogance he gives Draigo an opening to unleash a psychic assault and set Mortarion on fire, and then deliver the true name into his very soul. The true name isn't a word, it's more locked away knowledge that actually hurt Draigo when he tried to look at it. He unleashes the knowledge into Mortarion who then explodes, leaving a charred and barely alive husk. The GK eradicate the remaining Daemons and Draigo carves Geronitan's name into Mortarion's heart so that when the time comes and "The one" that Mortarion does fear, the one who will kill him, arrives he will have another weapon at his disposal, the name of Geronitan will be able to hurt Mortarion.

The audio ends with Draigo destroying Mortarion's shell and sending him back to the Warp.

was Sevatar.

It has to be garro.

Garro was a loyalist and the epitome of a noble 'knight.' So I don't see how him being Janus would 'destroy' the Grey Knights. Sevatar was traitor scum. One of the top brass of the traitors in fact.

Is either Garro or Omegon. For Garro the reasons are obvious. For Omegon because he is a two-faced god, hence Janus is an appropriate name, it would explain why Mortarion hates Janus so much and described him as "A brother who would betray his own for some half-imagined redemption." Garro wouldn't need redemption because he never did anything traitorous, Omegon did. It would also explain why the GK would be horrified by it, Omegon was a Traitor Primarch and treachery is a crime that can never be forgiven according to the Imperium they've grown up in.