Surprised no one's posted about this yet. From BL's 2016 advent calendar, it seems to have slipped under a few people's radars, and I had no idea it even existed until weeks after release. My eyes almost popped out when I stumbled on it and there wasn't even so much as a wet fart about it in the community.
Warning: SPOILERS AHEAD. You have been warned!!!
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The story is about Revuel Arvida, seemingly the last Loyalist Thousand Son we were introduced to in the short story Rebirth from the Age of Darkness anthology. After his adventures with the Vth Legion in Scars and Path of Heaven, this tale picks up very shortly after the latter. When last we saw Arvida he was in critical condition after almost single-handedly saving the White Scars' fleet by guiding it through the Emperor's beta Webway Portal, while simultaneously fending off the Fleshchange that haunted his Legion. When the White Scars fleet finally managed to limp to Terra an agent of Malcador boarded the Scars' flagship on orders from the Regent of Terra. When the Khan and his top Stormseers attempted to prevent his removal, the agent revealed that there was a cure for the Fleshchange, and Arvida could be saved.
*cue dramatic cliffhanger*

Seeing as though we'll all die of old age before Crimson King is ever released (seriously, it's been 7 years since A Thousand Sons was published), it was nice to see something of a coda to A Thousand Sons through Chris Wraight in Scars and Path of Heaven. He treats the XVth and their master with enough respect without having them steal the show from what's supposed to be a Vth Legion story.
Brief rundown:

*Malcador explcitly confirms there is no scientific cure for the Fleshchange. Malcador, and perhaps the Emperor, tried tweaking their genecode before the discovery of Prospero to help the XVth, but it was determined that nothing could be done to save the Sons. Simultaneously, Malcador acknowledges that Magnus saved his Legion with sorcery, not condeming this as heresy or forbidden because Magnus, and by extension the entire Primarch Project, was created by sorcery. Malcador never reveals how they knew it was sorcery that saved the Sons...

*The Khan believes that Arvida is singlehandedly responsible for saving the Vth and bringing them to Terra intact. The Primarch and the White Scars owe Arvida a blood debt for this and after fighting his way in, threatens to kill Malcador for his deception. I like Chogorian gratitude.

*Despite this, the Space Wolves guarding Terra are unaware of Arvida's presence. It was implied in Path of Heaven and confirmed here that measures were taken to ensure the VIth did not learn of his existence.

*Plot-shoehorn; first mentioned in the Ahriman trilogy, Magnus' psyche has been shattered into shards through a process that began with failing to warn the Emperor, then completed by the escape from Prospero. Each of these shards holds a fragment of Magnus' personality and, more importantly, his power. The Khan encountered one shard on Prospero in Scars, and another shard saved Numeon and the Salamanders survivors in Deathfire. Without spoiling too much of the Ahriman trilogy, from the start of the Heresy until 40k, Magnus has been running around as multiple personalities.

*Malcador has one such shard, left behind when Magnus deflated out of the Webway. Some deep foreshadowing and implications here; for one thing, despite the ending of Path of Heaven, Malcador never intended to cure Arvida, he needed Arvida to merge with the Magnus shard he had to create a 'shadow-Primarch' that would be able to fulfill Magnus' intended role of managing the Golden Throne, saving Malcador (for whatever reason) from having to take up that role himself. He was also prepared to euthanise him when it became apparent that even a powerful psyker Astartes would never be able to hold the essence of a Primarch.

*Arvida isn't really present in the story as the Fleshchange is in such a terminal stage that he's unconscious. His participation in the events is through a dying dream state, where he meets the aspect of Magnus' shard. Arvida reveals he's vaguely aware of the rest of the Legion's survival on Sortiarius and the coming Rubric, offering to work with Magnus to get to them.

*Taking it's cue from the Ahriman trilogy, the shard that Arvida meets seems to reflect his personality and temperament from between when he fled the Webway up to his decision to fight Russ - he is despondent and resigned, believing the coming of the Wolves to be justice for his crimes.

*This is the big reveal: in the dying dream-state Magnus is attempting to subsume Arvida, as Malcador had planned, but Arvida resists. The two instead merge in a trade-off of power. In the end, rather than turning into Spawn courtesy of the Fleshchange or exploding into warp energy because of failed possession, Arvida is transformed into someone, something, else. Described as being less than a Primarch but more than a mortal Astartes, appearing as both Arvida and Magnus, the new man introduces himself to the Khan as....Ianius.
So, it seems Arvida has gone from the very-obviously-but-unconfirmed founder of the Blood Ravens to the very-obviously-but-unconfirmed first Grand Master of the Grey Knights, Janus. His physical appearance described here practically matches his appearance in The Beheading, even down to having a busted eye. And of course I don't even to touch on the names and how Janus is properly written in Latin. Not entirely sure what to make of it. For one, I was hoping Arvida would make it all the way to the end of the Heresy to shuffle off and make the Blood Ravens. I thought the Knights Errant arc was going pretty strong and didn't need any 'outside interference'. I'm not particularly a Grey Knights fan but some of the community loved getting their panties in a twist over Janus' real identity.

Also of note, the identity of the tutelaries is brought into question. Previously, it was just assumed they were daemons, or at least something...else. But in The Talon of Horus, Path of Heaven, and Ahriman: Unchanged, their nature and identity were questioned, seemingly out of nowhere, I suppose leading up to Last Son of Prospero. The Magnus aspect repeatedly asks Arvida where his tutelary is, and even Arvida thinks it's a pretty random question to ask. The name of Arvida's tutelary is, plot twist ahoy, Ianius.
I am 110% ambivalent about this, but the community consensus seems to be that the tutelaries are actually the shards of Magnus. Whether because of his deal with Tzeentch or as a result of Warp time-travel hijinx is anyone's guess. Make of it what you will.
So, I'm super-stoked this came out. Good, but sad, to see some closure for Arvida.

Thoughts? Opinions?

I'm off to go find the remnants of the Thousand Sons fleet....