Spoilers here, so if you don’t know Kaius’ story skip this question. I don’t have anyone in mind for D14, but I could brainstorm a few possibilities, given a moment. He’s someone Kaius wants alive, but not allowed to speak or write. The idea that he’s a relative is one possibility. Another is that he’s a Karrnathi warmage who committed war crimes so vile he couldn’t be left at liberty (in part because other nations would demand justice) but Kaius wants to be able to retrieve him if there’s another war. A third even zanier possibility is that HE is Kaius I, and that the biggest secret of Karrnath is that Kaius III IS Kaius III, presenting himself to Kaius’ inner circle as his grandfather. If you want to get really deep into conspiracies, try this. Erandis arranged to have Kaius transformed. Because of the nature of this process, the vampire that transformed Kaius has direct influence over him, if s/he chooses to exert it. Unwilling to live with this threat hanging over him, Kaius I turned Kaius III into a vampire. In this theory, I’m asserting that a vampire can assert control over his direct progeny, but not over THEIR progeny. So K1 sires K3, and then arranges for K3 to send him under deep cover into Dreadhold. K3 poses as K1 posing as K3. His mission: to find and eliminate K1’s sire, so no one can dominate K1; once that’s done, he’ll free K1 from Dreadhold. In the meantime, Erandis is baffled by the fact that she can’t exert control over Kaius.
Personally, I like the idea of K3 posing as K1 posing as K3… but perhaps I’ve been watching too much Orphan Black.
Now, I just came up with this idea on the spur of the moment. But running with it a little further, you can get even farther out there and say Kaius III isn’t a vampire. Here’s the sequence of events. Kaius is turned into a vampire by Erandis. After being undercover for a time, Erandis sends him to replace K3 and take over Karrnath; she likes the idea of having a puppet on the throne. But K1 is no one’s puppet, and he has his own scheme. K3’s lover Etrigani is a deep-cover Deathguard agent, and knows rituals that can allow a living person to appear undead, a variant of the half-life techniques common among the Jhaelian clan. Together, Etrigani, K1, and K3 arrange to make it SEEM as though K3 is actually vampire K1. This “coup” – the idea that K1 has replaced K3 – is revealed to Morana and the other inner circle of Karrnath, and of course all of Vol’s spies. “K3” – actually K1 – is sent to Dreadhold, and made incommunicado, so there is no way for Vol to manipulate him; he can’t get out even if he wanted to. Etrigani and K3-posing-as-K1 want to destroy the vampire that sired K1 and to get as much information as they can about Erandis’ inner circle and her reach in Karrnath. Again, in this scenario Etrigani is an agent of the Deathguard, who have been trying to eliminate Erandis for centuries. But K1 is under close scrutiny by Vol. She doesn’t understand why she can’t control him, but he’s doing his best NOT to reveal his true identity. He’s too closely watched. He needs agents she doesn’t know… agents like the PCs.
The main thing I like about this is that most people who know Eberron know that Kaius III is a vampire and Kaius I. To negate both of these – not only is he actually Kaius III, he’s not even a real vampire – is a great way to catch people who think they know everything about the world offguard. And it helps solidify K3 and Etrigani’s relationship; it’s not that she loves him in spite of his being a vampire, something that’s an anathema to her people; rather, she loves him because they are working together to bring down Erandis, and it’s her skills that allow him to maintain his masquerade.