Girl Genius XX: Endothermic Life Forms Are Inherently Heretical

[Calemyr]

I think that has more to do with having to run the Empire and a mental imprint of his father looking over his shoulder all the time. Well, and with wanting to help Agatha of course.
So... Euphrosynthia having something to do with it is possible.
On the other Hand didn't she betray him somehow? Maybe this is why she did it. SK turns into a time eating monstrosity and she helps to seal him away.

It's a valid theory at this point, but we don't know much about anything, really.

We know that Voltaire was allied with him at one point. We know that Euphrosynthia was allied with him at one point, and enemies at another point, and both Marcellus and that Jaegar agree that they loved each other, but the Jaegar suggests that there was more at play than that. We know that he was allied with Van Rijn, that the muses were meant to be loyal to Valois, and that Van Rijn sided with Euphrosynthia in the breakup (with possible suggestions of an affair). We also know that one muse, the Von Pinn, was tasked by Valois with taking care of the next female Heterodyne - an objective she has done her best to interpret in creatively destructive ways. (Why do Bang and Von Pinn not get along again?) We also know that the lantern was legendarily used to save Valois from a "castle where the sun never sets".

We now also know that Van Rijn was obsessed with the Muse of Time, the one muse he did not create, and that he managed to capture her before he died. We also now know that at least one muse is working against her nature against Valois (assuming this guy is the real deal), actively saying she can't interfere while begging others to do so in her stead. We also now know that Valois (again, assuming) believes his last memory is of a "conjurer" sending him somewhere. Presuming this to be Van Rijn would be simple given the Sparks visionary accomplishments, but we don't have solid evidence yet.

My theory is similar to yours.
1 ) Valois the hero. Friend to Voltaire and Van Rijn.
2 ) Valois falls in love with a Heterodyne and continues pursuing heroism.
3 ) Valois's heroism and Euphy's heritage clash. Love gives way to rivalry, and then violence. War between the greatest houses in Sparkdom breaks out.
4 ) Van Rijn sides with Euphy. Maybe for love, maybe for SCIENCE!. The betrayal sends Valois out of control.
5 ) The lantern is employed to stasis lock Valois, freeing him from his torment. The last of the muses sits down to guard the tomb of her living master.
6 ) Van Rijn, feeling his years catching up with him, becomes more desperate to find immortality and captures the Muse of Time.
7 ) The muse cannot escape, but does not provide the needed help. Van Rijn dies in the pursuit, but the muse does not discover it.
8 ) Agatha frees the Muse of Time, finds the tomb, and watches as Violetta destroys the lantern to stop Lucy's minion from recovering it.
9 ) Valois is freed, his time stop rendering the betrayals still fresh in his mind.

I think it fits in everything we know, but there are still a lot of details to be revealed. Perhaps Euphy is the Muse of Time? It would even work in the larger narrative, as Lucy discovers Euphy's legacy while doing her own projects in the Kestle, then pushes further than she should and getting tangled up in something she couldn't get out of and Bill couldn't save her from. What comes back is a convoluted combination of Lucy and Euphy, two very powerful Sparks with no moral compasses. This would also mean that Von Pinn's duty should have been to find Van Rijn and protect the capsule containing the Muse of Time, but she didn't know this and instead read the instructions in a way she could actually pursue.

Ah, my bad. It's actually "the sun never sets" in the legend, not "never rises". Ah well.