Based on ambient dialogue in Nyedra that was along the lines of “a terrorling’s parasite could not survive in Zakiros”, I’ve been working under this assumption:
All kryptis have a parasite, and this parasite is how they possess people while retaining separate bodies.
But, what doesn’t make sense to me is why all kryptis would have a parasite if they are used only to possess targets, especially since the majority aren’t going to be leaving Nayos, although they seemingly can use their parasites on each other?
So here’s my guess: kryptis parasites were originally used to semi-benignly feed off the emotions of other beings, until Eparch weaponized them for possession instead.
It’s still a parasite, albeit a metaphysical one, so the targets would not be without consequences carrying one around in their heads. Finding a way to destroy the parasite would likely kill the kryptis but leave the host safe.
In terms of being weaponized, how well a possession goes for either party depends on both the strength and mental state of the target and the strength of the kryptis involved. Someone tired and emotionally vulnerable or with their guard down is easier to possess.
The strongest possession in this manner we see in-game is that of Cerus toward Isgarren.
Cerus is so strong, he can singlehandedly hold onto Isgarren in such a way that Isgarren is barely able to fight back at all, lasting long enough to hide himself in the World Spire before being taken over completely, and that’s even before Mabon dies since Cerus mentions he “let go of Isgarren” just long enough to let him see the moment.
(I also headcanon that different kryptis have different preferences for emotions, which in turn affects how much of an effect a parasite might have on their hosts. Peitha seems to particularly like feelings of defiance, for example.)
Unique Kryptis Possession Methods
Now, let’s talk about one of the underrated parts of SotO’s base storyline: the strategies of the kryptis invasion. Specifically: the way each Wizard was possessed in a manner tailored to them.
With the Wizard’s Court down, it was going to be only a matter of time before the rest of the tower (i.e. Astral Ward and Rift Hunters) fell. (If not for the Commander, of course.)
Dagda was actually possessed by not just one kryptis, but a whole “hive” of them led by Vanda, as identified by Peitha. Presumably, this was done as a precaution by the kryptis: Eparch must have thought that Dagda could’ve fought off any one kryptis on her own, hence the numbers strategy.
Notably, Peitha is pretty shocked when she realizes what was going on in the observatory, so this likely is an unprecedented way to possess someone.
Mabon was possessed by Asthenes, stated by Cerus to be one of his closest and dearest friends, who also appears to contradict everything I said just said about how kryptis possession works because you have to rip it out of Mabon’s body like a more traditional demon. Asthenes had tethered itself to Mabon’s soul: if one died, so would the other.
This sounds counterproductive, let’s put it this way: Asthenes was on a suicide mission, but it was guaranteed its success.
There was no certainty Mabon would sacrifice himself to purge Lyhr (which is the beginning of his death) especially since the Commander and the Heart of the Obscure changed the initial trajectory of the invasion. But Mabon is second only to Isgarren in rank among the Wizards. What if the Astral Ward had decided to purge Mabon first or find some other way to remove Asthenes from him? With the tethering to his soul, Asthenes guarantees that, no matter what, Mabon dies.
This method is likely a last resort type of thing, but with the guarantee of Mabon’s death, from the kryptis perspective, Asthenes was making a massive sacrifice to ensure victory.
Lyhr was possessed by two different kryptis. They went unnamed, as far as I can find. We don’t have as much information about this one, but it’s likely they set out to possess him as soon as he split, to force him to be unable to rejoin and ensure he would be weaker, thus weakening the Wizards in general.