Editing Sky Serpents (section)

==The Red Joy==
Sky Serpents delight in their work. They are laughing killers, for what better thing can there be than excellence matched against excellence?
However, all this is overlay upon the deeper impulses in the Sky Serpents psyche. The Red Thirst equivalent. It really needs a better name, but Red Thirst is just so good, you know?
Most of the time, it manifests as a blood-hunger in battle, anything to get the red flowing, but violence and destruction at their most primal level drive the marine.
An infamous example is a duel during Ullanor, when a Sky Serpents praetor, after wounding a warboss, discarded his power sword and proceeded to beat the Ork to death with his gauntlets over the course of several minutes.

Typically, this sort of behavior can be controlled through the odd sacrificial rituals of Tepectitlan, including the infamous heart sacrifice, but also including devotional acts of shedding ones own blood. (Ala Mesoamerica.)

These unsavory rituals, which range from sacrifice of live captives to what outsiders see as ritual desecration of enemy remains, contribute greatly to the legion's dark reputation.

Usually, this keeps everything in line, though unintentional massacres of civilians in combat zones are far from unheard of.

This contrasts with the phase II, which I'm thinking of as some sort of psychic curse put on the legion by Anshul. It involves aspects of a flesh change and the brother in question becomes increasingly feral. Historically, marines in this state sometimes were posessed by Khorne, but lately the Serpents have taken to sanctifing them with Seraphim, in a holy fire ritual. Unlike the Oceyolotl (who are the Gal Vorbak equivalent), the Seraphim are lost to their rage and are sent to die in combat.

The legion generally keeps them secret, though I would assume the other legions know that Anshul did something nasty to them. Like how people know that the black rage is a thing, but no details.

So basically, I'm thinking at Prospero, Xun and Anshul have a duel, an inconclusive one, but in the process, Anshul somehow marks the legion, tapping the aetheric energy of Prospero itself.

When a legionary falls into the Blood Hunger, they can further fall into the flesh-change. It starts like a curse of the wulfen deal, with the marine growing longer fangs and taking on more jaguar traits. They also become more animalistic. If it runs it's full course, they eventually devolve into chaos spawn, but they seldom make it that long, since they're sent into battle to die a good death like the Death Company.
However, this state leaves them open to possession, which the Asurans exploited on a few occasions, turning the assaulting Serpents back against their former comrades.
In response, the Serpents have taken to proactively possessing them with Seraphim, but this is top secret and does little to actually stabilize the brother's behavior.
They fight and kill until they join the Legions of the Damned as demonic Jaguars of the Emperor.

So basically, they've got the flaw of the Blood Angels. It drives them to greatness, but at the end of the day, there's a chance that they'll lose everything they've made of themselves and turn into a raging beast.

===Events on Prospero===
Maybe he lures Xun down into the reflecting caves on the pretext that he'll explain what is going on, since Anshul believes that once Xun knows about the true nature of Chaos, Xun will join him.

So Xun goes down to the reflecting caves, to meet Anshul and send his message.
The two forces meet under parley, like the Night Lords and the Dark Angels do in that one story. Across the surface and in orbit, the two legions stand facing each other. There's even some friendly chatter across lines, but mostly everyone waits.
Xun sends his message, which is part of the traitor plan anyway and then he has his talk with Anshul. Suffice it to say that it doesn't go well.
There's a grippy vision of a thousand different heresies, ones where the roles are reversed, ones where Anshul and Xun stand united against a mad Oramar, ones where Xun bears the sigil of the Wolf and Anshul has but one eye, ones where the Warmaster is named Hektor.
And then they start fighting, the planet itself becomes unstable.
The two legions tear into each other, the world quakes.
Both have to leave the planet and because of Anshul's trap, the price of Xun's escape is the curse on his legion. It's the taint of Prospero and they're forever intertwined with that world.
That's why Xun goes there, thousands of years later, to ascend.